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DIY Mini-Powerboat

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A cardboard boat powered by… soap? Really? Yes, really. Take a few of these common household items, help your kids follow the steps, and they’ll be rewarded with a little boat that has a surprising “engine.” And of course, we’ve got you covered (read on below) with the details of how it works.
A cardboard boat powered by… soap? Really? Yes, really. Take a few of these common household items, help your kids follow the steps, and they’ll be rewarded with a little boat that has a surprising “engine.”
What You’ll Need
  • Thin piece of cardboard, such as a cereal box

  • Scissors

  • Bar soap

What to Do

1. On your piece of cardboard, trace the shape of a house. At the bottom of the house shape, trace a small triangle notch.

2. Cut out this shape. 

3. Wet a small piece of bar soap and put it into the notch at the back end.

How to Use

1. Preparation: Rinse a bathtub or big sink with warm water to get rid of any soap residue.

2. Plug the drain and add just enough water so the shallowest area has ¼” water.

3. Watch as the boat takes off as soon as you place it in the water. If it doesn’t start right away, make sure the soap in the boat is touching the water. Or, you may need to drain and thoroughly clean the area again.

How It Works

When you launch the boat, the soap quickly begins to dissolve in the water. This forms a layer, or film, on the water’s surface. This film is made of soap molecules floating in the water like empty barrels.

As more soap dissolves, it pushes against this film. That push is what makes the boat go.

Finally, the boat will stop because the water’s surface is covered by the soapy film. Or, to think of it another way, the boat is surrounded by molecule barrels.

Extend the Fun

Younger kids: Experiment with different types of soap, such as dish detergent or liquid hand soap. Make sure to clean out the sink or tub before each test and fill it with clean water. And of course, like any good scientist, ask your child, “What do you think will happen?”

Older kids: Encourage your child to design other boats. See if she can design a boat that will go faster than the original or move longer. She can experiment with boats of different sizes and try cutting shapes from various papers or plastics. Talk about the advantages or disadvantages of different styles or materials. (This is especially great for the budding architect.)