Fact 1: Boiling is when liquids are heated above their boiling point.

Fact 2: Lots of gas bubbles burst over the boiling lakes' surface.

Given the two facts above, answer the question "What happens when liquids are heated?" with the following options: 
- Evaporation
 - chemical reaction
 - air is cooled
 - food is chilled
 - It expands
 - gas bubbles burst
 - flow of electrons
 - bubbles reduce


 

      gas bubbles burst   