Kids love to play around with sinking and floating, and you can keep them busy playing for a long time, and they learn science too!
Note: only do these experiments in a place where it’s OK if things get wet (outdoors, kitchens and bathrooms), have towels available, and know your kid will get wet!
Sink or Float is a great way for kids to experience the scientific method. All you need is a container filled with water, and some random household items to test. With each item you test, you can:
- ask a question – will it float?
- gather information – encourage the child to think about what they already know – have they seen items like this float before? have they seen boats or bath toys made of this material?
- form a hypothesis – ask them to predict what will happen
- test – what happens?
- share the results – for a young child, they just tell you verbally. An older child can learn how to chart to document their data
When we study this in our kids’ science classes, we want the kids to be able to test lots of variables to learn more about what kinds of things float and what things sink. Here’s our key ideas, then activities for exploring them…
- some materials float better than others,
- some shapes float better than others,
- lighter things generally float better than heavier things, but it’s not just about weight, and
- containers filled with air float better than if filled with anything denser than air.
For a toddler or preschooler, you’ll want to gather a bucket of items for them to test. For an older child, you could have them search the house for ideas for what to test. But be VERY CLEAR that they have to get your permission before putting any item in the water. You could basically say “when you find something, bring it to me, and we’ll gather info and form a hypothesis together, and then I’ll decide if it’s OK to actually test that hypothesis.” That way, they still have the learning of that discussion. But I can say “based on my prior knowledge, I’m 100% certain my cell phone would sink, AND I know the testing would harm it, so we’ll just mark phones down as sinkers.”
Here’s a list of ideas for materials to try: a cork, styrofoam or craft foam, plastic bottle cap, wax crayon, wood popsicle stick, metal lid, a shell, a rock, a raisin, a dried bean, oranges – peeled and unpeeled, a dry sponge, a feather, and so on… If you were designing the ideal tests, the samples would all be similar sizes and shapes (wood block, plastic block eraser, and so on) so the only variable here is material. But really, it doesn’t matter that much!
Note; It’s fun when they discover an item that does not float but also does not sink to the bottom – that’s neutral buoyancy and what happens when the density of the material is the same as the density of water.
Optional addition: Recycling. Show them an HDPE plastic container and an LDPE. Cut them up, dump them in water and shake. The high density sinks, the low density floats -that’s how they separate plastics for recycling.
Give your child something malleable hat they can shape in a variety of ways, like aluminum foil, or a waterproof clay (polymer or oil-based modeling clay, but not play-dough or ceramic clay that become a muddy mess in water). They can create a shape, see if it floats. They can add glass weights or pennies to it, or use a pipette to drip water in, to see how much weight it can carry before it capsizes.
You will read in books and online that if you make a boat from aluminum foil it will float and if you squash it up into a ball, it will sink. The foil boats work great and can even carry a lot of glass weights. But a wadded ball of foil will not sink. In order to get the foil to sink, you have to fold it in half, compress it as flat as you can, and repeat till it’s the absolute tightest densest square of foil you can make. But you could experiment with clay – some clay will sink if balled up and float if shaped right. Play Dough to Plato has a great description of how to do this process with kids. (Find more details at Teach Engineering and Mr. Williams 6th grade science blog.)
You could also offer some toy boats at this station, and then demo to the children that if they set them in with the flat hull facing down, they float, but not if you flip them on their side or top. You could also show how they float when they are empty (which of course means full of air), and then what happens if they’re filled with glass weights, or what happens if they start taking on water.
Dancing Raisins: Simple, cheap and easy way to delight small children, that also shows that shape matters…. Pour a cup of carbonated soda. Throw in a few dried beans – they sink. Add raisins. The bubbles of gas collect in the ridges of the raisin until it floats to the surface, then they pop and the raisin sinks back down… over and over again.
Do heavy things always sink? Can you make heavy things float?
Loading Up a Boat: Offer a container (or multiple containers) that will float – things like toy boats, plastic cups, plastic bowls, styrofoam take out containers. Offer weights – marbles, glass stones, or pennies can work.
First have your child drop a weight in the water. It will sink to the bottom. Then have them put the weight on the “boat.” It floats! Kids learn that if a weight is put on a wide platform, they don’t sink. (Little kids won’t be able to understanding the physics of the idea that the weight is spread out over more area, but they can experience it.)
Have them add more and more weights till the boat sinks. They’ll discover it’s fine, and then suddenly – not fine! The second that water starts to get in, the boat sinks quickly.
Sidebar Math Activity: Kids count the number of glass marbles that their boat can hold before it sinks. We had tally sheets where they could keep count and compare which vessel could hold the most glass weights. They can see that each type of boat has its maximum capacity .
What’s Inside the Container Matters
Have three identical containers that are airtight. Leave one empty, fill one halfway with water, and one all the way with water and test which floats best. You could use water bottles, clear plastic containers, or balloons. (Note: filling a balloon halfway with water and halfway with air is tricky. First, make a water balloon and an air balloon so you know what the size is your aiming for. Then fill a balloon with about half that much water, then insert a straw, pinch the opening of the balloon tightly around the straw and use the straw to blow in air till it’s a little bigger than you want it. Pull the straw out, keeping the opening pinched tight to hold in the air, then tie it off.)
If you’re reading What Floats in a Moat (see below), you can re-create the experiment from the story by having three identical plastic containers, ideally barrel shaped – one filled with buttermilk (or water mixed with some white paint so it looks like milk), one half-full, and one empty so kids can test what happens.
They’ll discover that a container filled with air floats (air is less dense than water), a container filled with water sinks (because the weight of the water plus the container makes it denser than the water), and the half full container bobs – the water in it is all under the level of the water – the half filled with air is above the level of the water.
Does it Matter What They’re Floating On?
In class, we do this experiment, but I don’t think I would do it at home during quarantine period, just because it can use up a lot of carbonated water or a lot of salt and I know we’re all trying to minimize trips to the grocery store. But here it is, just FYI:
Exploring Buoyancy: We put out three tubs – one filled with fresh water, one with salt water, and one with carbonated soda / seltzer water. Kids were encouraged to test a bowl in each of the three tubs, slowly adding stones to it to see which kind of liquid best floats their boat. (Note: You need to use a LOT of salt to notice the difference in buoyancy. Like at least 1/4 cup salt per cup water.)
Egg Float. Provide a hard boiled egg, a cup of warm water, a dish of salt and a spoon. Let kids discover that if they place the egg in the water it sinks. Encourage them to stir in a spoonful of salt, then test it again. How many spoonfuls of salt do they need to add to make the egg float? (Answer… it takes a LOT of salt… close to 1/4 cup) Every time a child finished this experiment, we’d dump their results into our tub of salt water (see above) so it got a lot saltier as the day went on.
After kids have explored all these elements of sinking and floating, then issue a challenge: Engineer a Boat that Floats. In class, we offer a “ship building factory.” Click on that link for the full tutorial post on “Building a Boat that Floats.”
Here’s some pictures of boats that we have made:
Bubble Blowing Art
Add food coloring to bubble solution. (don’t add much color or you’ll dilute the bubbles!) Kids blow bubbles at paper, and as the bubbles pop, they make splashes of color. We’ve tried it with the paper laying horizontally on a table, and vertically on the easel, and prefer the easel.
Create Discovery Bottles
Fill water bottles with a mix of colored water and vegetable oil, then add some sand, small rocks, and small sea shells. Then seal the bottles. The bottles create their own slow motion waves when tipped or shaken. (Point out that the oil floats on top of the water, because it’s “lighter” (less dense) and the sand and rocks sink because they’re heavier than the water.
Other Art Ideas: You could try Floating Chalk Art, where a child scrapes chalk dust onto the surface of a tray of water then takes a print of it. Or you could try paper marbling.
If the book title is linked, that means there is a Kindle edition (and it’s an affiliate link so I get a small referral fee if you buy anything.) If there’s a YouTube link, that’s to a video of the book read aloud. Some of these are also available in e-book from your local library.
What Floats in a Moat? by Berry and Cordell. YouTube. This is not just a book about sinking and floating, it’s also a good book about inventors (Just click on that link to learn more about this book and other great books!) And it’s a fun rollicking read-aloud. Archie the Goat wants to cross a moat. He muses that a barrel might float. So he builds a boat with a barrel full of buttermilk – it sinks. He builds a second boat with an empty buttermilk barrel. It floats but then capsizes. Then he builds the third boat, the S.S. Ballast with a barrel half-full of buttermilk. It’s perfect! Read the book, then talk about displacement. Caveat: I think the text of the book doesn’t really make it clear what’s happening with the third barrel – you may want to say something to the kids when reading it to clarify that this time the hen only drank half the buttermilk, so the barrel is half full.
Who Sank the Boat? by Allen. YouTube A great book for even very young audiences (age 3 – 7.) One by one, a cow, a pig, a donkey, and a sheep climb into a boat. Will it sink? When will it sink? The suspense builds and builds. Then the final creature steps in – a tiny mouse. And the boat sinks. Fun read, and interesting examination about the cumulative effects of things. (Like the straw that broke the camel’s back.) You could also re-create this with a container and plastic animals… how many can fit before it sinks. Make sure they understand that it’s not that the mouse was heavy… it was about all the animals added together.
Captain Kidd’s Crew Experiments with Sinking and Floating by Weakland. YouTube Silly approach to a non-fiction science book about density, buoyancy, and other science concepts. Good for 6 – 8 year olds.
Swim, Swim, Sink by Harney. YouTube. This is a story about three baby ducks – two of them swim, one sinks. OK, this is not a science book – it never explains why one duck keeps sinking. But it is an absolutely delightful and silly book that would be great to read to 3 – 4 year olds, and the duck does have to get very inventive to figure out how to not sink.
Things That Float and Things That Don’t. YouTube. This is not something I would read aloud to 3 -5 year olds – too complex. But, it’s a FABULOUS book for 6 – 10 year olds, especially in a home school setting, where you and your child could actually walk through each of the experiments together. Fun illustrations of a boy, a girl, and a dog, plus the fact that the book is addressed to the reader – inviting them to try everything out (“You could have fun guessing which things float…. fill your sink about halfway…”) make this an engaging non-fiction read. Guides kids through a series of observations and experiments with household objects to learn about density.
Big Machines Float! by Veitch. If you have a child who LOVES big trucks books about construction equipment, they’ll like this book, which is basically just a collection of pictures of big boats at work: ice breakers, floating cranes, galleons, cargo ships, and cruise ships. Ages 3 – 6.
Float by Miyares. Like nothing else on this list. For ages 4 – 8. A wordless book with lovely, quiet, contemplative illustrations. A boy folds a newspaper into a boat. He goes for a long walk in the rain by himself (may appeal to those who like a more nature-based, free range style childhood) and sails the boat on many puddles. Eventually the boat is destroyed. He is sad and his father comforts him (nice depiction of emotions and support) and then helps him fold a new boat which he goes out to enjoy on a now beautiful sunny day. It’s a lovely wordless book, but whether it would work for your audience all depends on how you “read it”. It helps to review it by yourself first so you can figure out what to say to capture the magic of the book. There’s a YouTube video that shows the pages. You could pause the video to discuss them.
You can read reviews of the following books in my old sink and float post, but I’m not including details here because they’re not available in e-book or video format.
Let’s Try It Out in the Water : Hands-On Early-Learning Science Activities by Simon, Fauteux, and Cushman. Floating and Sinking (Start-Up Science) by Challoner. Floating and Sinking (How Do Things Move?) Sinking and Floating (Simply Science)by Rosinsky. The Magic School Bus Ups And Downs: A Book About Floating And Sinking. Making Things Float and Sink by Gibson.
Also check out this post on books about the beach and this one on books about the ocean and fish. And PBS’ Whatever Floats Your Boat, an activity guide from Cat in the Hat Knows All About That.
This 5 minute Sci Show kids is good for age 5 and up: https://www.youtube.com/watch?v=eQuW8G2QV_Q or this Sesame Street is good for 3 – 4 year olds, but it’s pretty slow moving… 22 minutes…https://www.youtube.com/watch?v=KmoFS2WYIzA) Uniland has videos where they test fruits or vegetables. There’s no words, so you can watch these with your child, and each time they show an item, ask them to make a prediction: https://www.youtube.com/watch?v=aMbQSe6IdBU. Or this video shows a dad taking a kid through an experiment of making a clay boat – https://www.youtube.com/watch?v=f3ZQCWnxVzg. Note, he used “play-dough” – it works better with a clay that won’t get slimy in water.
Enjoy your experiments with sinking and floating!
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