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Adding to a Full Cup

When is a glass full of water really full? You may be surprised at how much you can add to a full glass without overflowing the water.

For this one, you will need:

  • a glass full of water
  • lots of coins
  • a plate or saucer

Fill the glass full of water and place it on the saucer, to catch the water when it finally overflows. If the glass is not completely full, use a cup to add water up to the top. Now that it is full, carefully drop a coin into the glass. How many coins do you think you can add before the water flows over the edge? Think about this, and come up with an estimate before you actually try it.

Once you try it, you will probably be very surprised. As we have seen in other experiments, water molecules stick together and form a skin-like surface tension at the surface of the water. This surface tension keeps the water in the cup, letting it bulge upwards without overflowing. Even though the coins seem to be large, they are very thin and do not take up much space. As long as you add the coins carefully, you can add quite a few coins before the water finally spills out of the cup.