Materials Required
  • Two identical bottles
  • Water

How to do it

  • Fill one bottle with water.
  • Leave the other bottle empty.
  • Cover the two bottles with the stretched rubber cork.
  • The next day the rubber over the bottle of water wil be bulged slightly.

Explanation

  • The molecules of water are in constant motion. Sometimes they jump out of the water and into the air.
    The presence of the water molecules increases the pressure of the air slightly, but enough to cause a slight bulge in the rubber.