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Let’s talk about one of the simplest, cheapest, and best science activities of all time: the classic What Floats in Water? experiment (also known as Sink or Float).

Not all water play has to be pure chaos and splashing (although we love that too). Sometimes we can sneak in a little legitimate brain-building while the kids think they’re just having the best afternoon ever. This is one of those magical activities that feels 100% like play, but quietly teaches real science concepts like buoyancy, density, and forming hypotheses. The best part? Your child probably won’t even notice they’re learning.

Ready to turn your kitchen, bathtub, or backyard into a floating laboratory? Let’s dive in (pun absolutely intended).

Why This Activity Is Pure Gold

The What Floats in Water science experiment is a perfect afternoon activity for the heat of summer for multiple reasons. My favorite are that it’s free, beats summer boredom, and teaches a child some true science concepts. Here’s a list of all the reason:

• Takes 5 minutes to set up
• Costs basically nothing (you already own everything)
• Works for ages 2–9 (and honestly, big kids and adults get into it too)
• Naturally leads to rich conversation about why things behave the way they do
• Cleans up is a breeze
• Leads into more outdoor water play

And what we’re secretly teaching is density, without having to say the word. Instead we focus on: what floats in water. We can still use proper scientific vocabulary, such as density, buoyancy, and hypothesis but we’ll get to the kid-friendly way to explain that later.

Let’s get started!

Pre-Load the Learning

Kids learn best when new ideas feel familiar. Before you dump a pile of random objects next to a tub of water, gently introduce some of these concepts. Here are several different ways to do so:

1. Read a few water-themed books in the days leading up (suggestions below).
2. Casually ask over breakfast, “Hey, why do you think boats don’t sink but rocks do?”
3. Go on a nature walk and notice ducks floating, leaves drifting, and sticks bobbing.
4. Watch a 2-minute YouTube clip of huge cruise ships or aircraft carriers (kids lose their minds when they realize something THAT big floats). This is also a great way to end the What Floats in Water science experiment.

These tiny seeds make the actual experiment feel like an exciting payoff and will add to their overall learning. Although random is sometimes fun by including these enriching ideas leading up to the science experiment you’ll be nurturing bigger concepts than if it was to be kept as a random activity.

Books We Love About Water & Floating

• Water Is Water by Miranda Paul This lyrical picture book beautifully traces the water cycle through a child’s everyday observations, turning rain, steam, snow, and more into gentle poetry that feels like a warm storytime hug. Perfect for helping preschoolers see water’s many forms without feeling like a lesson.
• Over and Under the Pond by Kate Messner With stunning illustrations, this book explores the hidden world of life above and below a quiet pond, showing how animals, plants, and water interact in peaceful harmony. It’s a wonderful way to spark curiosity about nature and ecosystems right at the water’s edge.
• Who Sank the Boat? by Pamela Allen In this hilarious, repetitive classic, a group of farm animals climb into a tiny boat one by one until—splash!—someone makes it tip, and kids love shouting the culprit’s name each time. The silly rhyme and predictable pattern make it irresistible for read-aloud giggles while subtly introducing balance and buoyancy.
• Float by Daniel Miyares This nearly wordless gem follows a little boy’s paper boat on a rainy-day adventure through puddles, streets, and imagination until it returns home transformed—pure magic in soft watercolor illustrations. Ideal for the youngest children who can “read” the story through pictures alone and feel the wonder of floating.
• Hey, Water! by Antoinette Portis Bold, simple text and bright artwork celebrate water in all its playful and powerful forms—from drinking glass to ocean wave to tiny dewdrop—inviting kids to shout “Hey, water!” back at every page. It’s short, engaging, and perfect for building early vocabulary around water’s many roles.
• Water Can Be… by Laura Purdie Salas Playful rhymes and vivid photos show water as friend, force, home, and artist (think frozen sculptures, thirsty plants, and crashing waves), making abstract ideas feel concrete and exciting. Great for toddlers and preschoolers who love pointing and naming everything they see.
• A Drop Around the World by Barbara Shaw McKinney Follow one single water drop as it travels through rivers, clouds, animals, and people across the globe in this poetic journey that ties the water cycle to life everywhere. The rhythmic text and detailed illustrations make it a lovely bridge to bigger-picture thinking for slightly older preschoolers.
• The Magic School Bus at the Waterworks by Joanna Cole Ms. Frizzle takes the class on a wild ride through pipes, reservoirs, and the water cycle in her classic zany style, packed with facts, humor, and unforgettable visuals. A fun, slightly longer read that sneaks in tons of real science while keeping everyone laughing.

These books pair beautifully with the sink-or-float experiment—read one or two beforehand to plant ideas, and afterward to connect what they just discovered to bigger water stories. Your little one will start spotting “floating” and “sinking” everywhere!

So… What Floats in Water? (The Super Simple Answer)

Here’s the concept in kid language:

“Some things are lighter than water, so they float on top like a beach ball.

Some things are heavier than water, so they sink like a rock.

But it’s not really about how heavy something is—it’s about how much stuff is packed into it (that’s called density!).”

A cruise ship is WAY heavier than a penny, but it floats because it’s mostly filled with air and spreads its weight out over a giant area. A penny is tiny but packed solid with metal, so down it goes.

We grown-ups know this is Archimedes’ principle (an object floats if it displaces water equal to or greater than its own weight), but for young kiddos we just stick with “less packed = floats, more packed = sinks.”

Materials You Need (Spoiler: You Already Have Them)

1. A container with water
   • Indoor options: storage bin, large mixing bowl, sink, or bathtub
   • Outdoor options: kiddie pool, water table, or big bucket → Bigger is genuinely better here. A kiddie pool lets you test that giant stick your child drags out of the woods determined to show you that bigger items sink.
2. A bunch of safe, waterproof objects: Great ones to gather:
   • Coin, key, paper clip, screw, marble (classic sinkers)
   • Wine cork, Lego piece, plastic toy, ping pong ball, feather, craft pom-pom (classic floaters)
   • Stick, leaf, flower petal, pinecone, acorn, sweet gum ball, pebble
   • Spoon (wood vs metal is fascinating)
   • Small plastic bowl or cup (show how an “empty” thing floats but fills and sinks)
   • Ice cube (floats—then melts!)
   • Grape, blueberry, cherry tomato (mini food science!)
   • Toy car, toy boat, rubber duck (obvious but fun)
   • Foil—make a ball (sinks) then a boat (floats!)

Pro tip: Let your child hunt for items. The ownership makes it 10× more fun.

3. Two different-colored towels or two baking sheets labeled “Float” and “Sink”
4. Dry towels for the inevitable drips (and for you).

Step-by-Step Instructions

1. Set the stage Lay towels out, fill your container about ⅔ full, gather objects in a basket.
2. Introduce the game “We’re going to be scientists today! We get to guess—will it sink or float? Then we test it and see who was right!”
3. Model once Hold up a cork: “I think this will float because it feels light and has tiny holes with air. Let’s see!” Drop → floats → place on blue towel. Hold up a coin: “This feels heavy and solid. I bet it sinks.” Drop → place on green towel.
4. Hand over control and let your child pick an object, ask “What’s your hypothesis?” (or “What’s your guess?” for younger kids), then let them drop it and sort it onto the correct towel.
5. Repeat until every object has been joyfully launched into the water (and probably until your child is soaked and grinning).

Turning Play into Actual Learning (The Easy Way)

You don’t have to lecture. Just weave in a few natural questions:

• “Whoa, the giant stick floated but the tiny coin sank! That’s so weird—why do you think that happened?”
• “Both the Lego and the marble are hard, but one floated and one sank. What’s different about them?”
• “The pom-pom floated, but when it got totally soaked it started to sink a little. What changed?”
• “Remember the cruise ship video? It’s bigger than our house but still floats. How is that possible?” (Leads perfectly to the idea that big things full of air can float.)

When they group everything on the towels, stand back and ask them what they notice:

• “Almost everything made of wood floated! And almost everything metal sank!”
• “Plastic is tricky—sometimes yes, sometimes no.”
• “Things with air trapped inside (cork, pom-pom, ping-pong ball) floated great!”

Then drop the golden nugget casually:

“It’s not really about big or small, or even heavy or light. It’s about how packed the stuff is inside. Scientists call that density. If it’s less packed than water, it floats. If it’s more packed, it sinks.”

Say it once, then move on. They’ll remember because they saw it with their own eyes.

Level Up: Further Experimenting

Once they “get it,” watch the magic happen—they’ll start testing their new theory everywhere.

My kids have:

• Run to the woods for the biggest stick they could carry (floated!)
• Tested every rock in the yard (all sank)
• Made foil balls vs foil boats (mind officially blown)
• Filled plastic Easter eggs with coins vs air
• Tested a leaf (floated) then crumpled it into a tight ball (still floated, but barely)

Have a few surprise items ready:

• A AA battery (sinks) vs a fat crayon (floats)
• Dry spaghetti (floats) vs cooked spaghetti (sinks)

Doing This Experiment Indoors (Mess-Contained Edition)

Rainy day? Tiny apartment? No problem!

Use a big storage bin, bathtub, or the kitchen sink. Put an old sheet or beach towel underneath to catch splashes. Keep the objects small-to-medium (no tree-trunk-sized sticks indoors, please). A cheap dollar-store tray nearby also works beautifully and contains everything.

What If My Child Keeps Guessing Wrong? (And Other Tricky Moments)

It’s totally normal if your kid insists a giant rock will float or that a feather will sink. Don’t correct their guess ahead of time—just smile and say, “Let’s let the water decide!” The surprise when they’re wrong is where the real learning happens. If they get upset about being “wrong,” reframe it fast: “Scientists guess wrong all the time! That’s how we figure new things out. You just made a discovery!”

Other common moments:

• They want to test the cat/dog/remote control → Gently redirect with, “That one likes to stay dry. How about this stick that’s the same size?”
• Everything ends up floating because they only picked light toys → Pull out a surprise sinker from your pocket (coin, screw, marble) and let the learning continue.
• They start throwing instead of dropping → Turn it into a mini-lesson: “Throwing adds extra force. Scientists drop gently so we only test floating vs. sinking.”

Fun Water Facts to Share

1. A giant cruise ship weighs more than 50,000 cars, yet it floats because it’s mostly hollow and full of air.
2. Your body floats really well in the Dead Sea because the water is extra salty (and extra dense).
3. Ice floats on water because air is trapped inside—that’s why icebergs float and why your iced tea doesn’t have ice at the bottom.
4. Ducks float because their feathers trap tiny air bubbles like a built-in life jacket.
5. About 60% of your body is water, which is one reason you can float pretty easily when you relax in a pool.

Five More Water Activities to Continue the Learning

Dancing Raisins/Grapes – This is a perfect activity to do the same week as the What Floats in Water science experiment because they explore the concept: Density. Drop raisins into clear soda. Bubbles stick to them, carry them up, pop, and they sink again—over and over. This keeps my boys entertained for at least 30 minutes.

Foil Boat Challenge – Give everyone a square of aluminum foil and see who can make a boat that holds the most pennies before sinking. Engineering + buoyancy in one.

Iceberg Excavation – Freeze small toys in a bowl of water overnight. Give kids warm water droppers and salt to “rescue” the toys.

Magic Pepper & Soap Trick – Sprinkle pepper on water (it floats!), touch the center with a drop of dish soap and watch the pepper race away. Teaches surface tension.

Walking Water – Put three glasses in a row—empty, full of colored water, empty. Add paper towels as “bridges” and watch the water “walk” across overnight. Capillary action made magical.

There you go, everything you need to turn ordinary water into an afternoon of wonder, giggles, and real learning. So grab those random toys, fill something with water, and ask the best question ever:

“What floats in water?”

Your little scientist is about to tell you—and they’re going to have the absolute best time proving it.

Read Next: Preschool Water Activities

Sink or Float?

The What Floats in Water science experiment never gets old because every single time a child drags in a new treasure from the yard (“Mom, will THIS float?!”), the excitement is brand new. It’s the perfect blend of messy outdoor fun, genuine science discovery, and those proud “I figured it out!” moments. Leave a comment below sharing your water play experience with your child!