Float or Sink Challenge: A Simple STEM Activity

A simple, hands-on STEM activity to explore buoyancy, density, and water

What floats? What sinks? And why?

You probably already know that a rock sinks and a leaf usually floats. But what happens when you start testing objects that aren't quite so obvious? A spoon, an apple, a stick, a plastic lid, or a piece of bark. Before long, a simple bowl of water can turn into a little science investigation.

The Float or Sink Challenge is an easy, low-prep STEM activity that invites kids to predict, test, observe, and look for patterns using objects you already have around the house or can find outside. And the best part? You don't have to explain everything before you start. Let them wonder first.

What You'll Need

You only need a container of water and several objects to test. A clear bowl, bucket, baking dish, or even a small kiddie pool can work. If you're outside, a shallow container can make this especially easy.

Try gathering 5–10 items such as:

• A leaf

• A pine needle

• A small rock

• A spoon

• A stick

• A cork

• An apple

• A small toy

• A plastic lid

• A nut or acorn

You don't need all of these. Use whatever you have available.

Start With a Prediction

Before putting anything in the water, ask: "Do you think it will float or sink?" Have your child make a prediction for each object.

You can keep it very simple:

Object	My Prediction	What Happened?
Leaf	Float
Rock	Sink
Stick	Float
Spoon
Apple

Younger children can simply point to FLOAT or SINK or say it out loud. Older children can write down their predictions and results. The important part isn't getting every prediction right. It's making a prediction and then testing it.

Test Your Predictions

Place each object gently into the water. Watch what happens.

Does it:

• Float on the surface?

• Sink straight to the bottom?

• Slowly sink?

• Start floating and then become waterlogged?

• Sit partly above and partly below the water?

• Move or roll before settling?

Take your time and really watch. Sometimes the most interesting observations happen after the object has been sitting in the water for a while. For example, a piece of bark might float at first but become heavier as it absorbs water. A leaf might float quietly until a small drop of water weighs it down.

What Makes Something Float or Sink?

This is where the investigation gets interesting. Whether something floats or sinks has a lot to do with density and buoyancy. An object that is less dense than water can float, while an object that is more dense than water tends to sink.

But an object's shape matters too. Think about a boat. A boat can be made from materials that would normally sink, but its shape allows it to displace enough water to create an upward buoyant force that supports the boat.

You don't need children to memorize the science behind this. Instead, let them discover the idea through the experiment:

Why does a tiny rock sink while a much larger piece of wood can float?

That's a much more interesting question to investigate than simply telling them the answer.

Look for Patterns

Once you've tested everything, spread your objects out and look at the results.

Ask: What do the objects that floated have in common? What about the objects that sank?

You might notice that:

• Some lightweight objects float.

• Some heavy objects sink.

• Some large objects float.

• Some small objects sink.

• Some objects behave differently depending on their shape.

And here's a fun one:

Does something have to be light to float?

An apple is much heavier than a pine needle, but it can still float. This is a great opportunity to let your child come up with their own explanations.

Try Something Unexpected

Now find an object you aren't sure about. Maybe a metal spoon. Ask: "What do you think will happen?"

Let your child make a prediction before testing it.

Then try changing the way you place it in the water. Does it make a difference? This can lead to some really interesting conversations about shape, air, and how an object interacts with the water.

Take It Further: Can You Make Something Float?

Now turn the investigation into a simple engineering challenge. Give your child a piece of aluminum foil. Ask: "Can you make a boat that floats?"

Start by simply crumpling the foil into a ball and placing it in the water. What happens? It sinks.

Now flatten it out and shape it into a little boat. Try it again. What changed? The material didn't change. The shape did.

The Penny Boat Challenge

Now add another challenge:

How many pennies can your foil boat hold before it sinks?

Have your child make a prediction.

Then add pennies one at a time. Count them carefully. When the boat sinks, record the number. Then try again.

Can you redesign the boat to hold more?

Maybe make the bottom wider. Maybe make the sides taller. Maybe change the shape completely. Test it again.

Challenge:

Can you beat your first design? This turns the activity into a simple introduction to engineering design:

Build → Test → Change → Test Again

A Little More Science: What About Surface Tension?

You may also notice something interesting when you gently place certain lightweight objects on the water. Instead of immediately sinking, they can sometimes sit on the surface. That's partly because of surface tension, the behavior of water molecules at the surface.

Try this with a very small, lightweight natural object, such as a pine needle. Place it gently on the water. What happens? Now gently push it below the surface. Does it behave differently? This is a great example of how one simple activity can lead to several different questions.

Take It Outside

This activity doesn't have to happen at the kitchen table. Take a bowl, bucket, or shallow container of water outside and gather a few things from nature.

leaf → pine needle → small stick → rock → acorn → piece of bark

Before testing each one, ask: "What do you think?" Then test it.

You can look for natural objects that you think might surprise you.

Questions to Keep the Conversation Going

You don't need to ask all of these, they are there to give you a bank of questions you can use. Follow your child's curiosity.

• Why do you think this one floated?

• Why did that one sink?

• Were your predictions correct?

• Did anything surprise you?

• Does size determine whether something floats?

• Does weight determine whether something floats?

• What happens when an object gets wet?

• Can you make a sinking object float?

• Can you make a floating object sink?

• Does changing the shape change what happens?

• Which boat design held the most pennies?

• What would you change about your design?

The Point Isn't Getting It Right

One of my favorite parts of simple STEM activities like this is that there doesn't have to be a right answer before you begin. Make a prediction. Test it. Notice what happened. Then ask another question.

Maybe your child predicts that the apple will sink because it's heavy. It floats. Now you have a new question: "Why?"

That moment of surprise is where the learning begins. You don't necessarily need to jump in with an explanation. Give them a chance to think about it.

What do you notice? What do you wonder?

A Simple STEM Adventure

The Float or Sink Challenge can be as simple as a bowl of water and a handful of objects, or you can spend an afternoon designing foil boats and seeing how many pennies they can carry.

There's no need to make it complicated.

Start with a question. Make a prediction. Test it. See what happens.

And if your child wants to keep going, follow the next question.

That's the beauty of hands-on learning.

Observe. Wonder. Explore. Discover.

Extra Information

Don't worry, you don't need to memorize this information. It is only here if you want to go into more detail.

Looking for more simple September activities? Visit the September Activities Page for more nature-based ideas, simple STEM adventures, and things to do together.