The water increased the pressure under the lid. The jar shrunk with the warm water. The water acted as a lubricant between the glass and the metal. The metal lid expanded under warm water.
I gave my students this multiple choice question: The metal lid on a glass jar is hard to open, so it is held under warm, running water. What causes the jar to open easily after it was held under the water?

Only one answer is correct among these 4 choices:
- The water increased the pressure under the lid.
- The jar shrunk with the warm water.
- The water acted as a lubricant between the glass and the metal.
- The metal lid expanded under warm water.

Could you tell me which one is correct?

The metal lid expanded under warm water.