Make Things Sink, Then Float

You will need

  • Modelling clay or blue tag
  • Marbles (if you don’t have marbles, use small rocks)
  • Water
  • Glass bowl or a large glass jar

What to do

  1. Fill the glass bowl or jar with water.
  2. Drop the marbles into the water. They quickly sink to the bottom. Roll the clay into a ball and drop it into the water as well.
  3. The clay also sinks like the marbles.
  4. Remove the marbles and the clay ball from the water.
  5. Flatten the clay as much as possible, then shape it to make a boat and place it into the water. Now it floats!
  6. Add one marble as a cargo. The boat settles lower, but still floats.
  7. Add more marbles, one at a time. How many can you add before your boat sinks?


Large ships float on water, even though they are very heavy.  However, a small object like a marble sinks. The mass of the object does not matter. Whether or not an object floats depends on its density and how much water it “displaces”, or pushes aside.

When the clay is shaped like a ball, it does not displace much water and it sinks. When shaped like a boat, the clay has a bigger area and displaces more water which makes it float. When adding your cargo of marbles, the boat settles lower to displace more water and carry the extra mass. When adding too many marbles, the boat will settle below the water line and sink.

This is important for ships. Too much cargo can lower them too close to the water line and make them vulnerable to sinking.

You can watch this experiment as a video here:

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