What you Need
- Glass of water
- Tray (in case of spills!)
What to do
1. Fill two glasses with ice: one with a few cubes in it and one with so much ice it stacks up to the brim
2. Top up the glasses with water until the water level is just above the brim, being held there by surface tension
3. Leave the two glasses for the ice to melt - will the water level go up, down or remain the same?
What may happen
The glass with only a few cubes in it should have remained at exactly the same water level. But the glass with the ice stack up will have overflowed.
Why does it happen?
This is because the ice floating in the first glass has displaced exactly the same volume of water that it is made up of, so as it melts the overall volume shouldn't change. For the glass with more ice, the ice cubes are resting on each other as they reach the bottom of the glass rather than floating on the water, so this displacement is not taking place. Therefore as the ice melts the glass overflows.