Float the Boat!

Get wet, build a boat, have a contest, and learn some science!

Why does a boat float?

Have you ever asked yourself why boats and balloons float but pennies sink? Sure, they may weigh the same, but a balloon and a penny act so very differently in the water.

That's due to density! If two objects take up the same amount of space, and one is heavier, that means it is denser, and if the object is denser than water, it will simply sink. If it is less dense than water, then it will float. Since a penny is small, but heavy for its size, it sinks. A balloon, on the other hand, is big but also very light for its size, so it floats. A boat floats because it takes a lot of space, but the middle is all air – which is not heavy at all. This sounds complicated, but it is easy to understand if you try it out.


-A sink, plastic tub, or wading pool filled with water at least three inches deep. If you are worried about spilling water, do this outside!
-Aluminum foil
-Paper clips
-Popsicle sticks or twigs
-Rubber bands
-Pennies or pebbles
-Random items to test in the water

What floats?

Try some things that are okay to get wet and see if they float or not. Some ideas: Legos, a pencil, a marker, some different kinds of fruit, a stick, a plastic toy. Which floated? Which sank? Did any surprise you?

Make a Boat

Use aluminum foil, Popsicle sticks, rubber bands, and paper clips to shape a boat that you think will float. Test your design in the water – does it stay up? Tip over? Can you change your design to float better?

Which pirate ship do you think will hold the most stuff without sinking? Can you think of a shape that might work better? 

Float the Boat Contest:

Everybody puts their boat in the water. Add pennies or pebbles to your boats, one at time. Whichever boat stays afloat longest wins! Which design worked best? Why did some boats sink faster than others?

Try these resources to learn more about boats and density!

Book: How Big Is Big?

eBook: Building Boats that Float

Video: All About Boats and Ships

To learn more about density, try these interesting experiments in SIRS Discoverer.