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

Fact 2: Vapor pressure is about liquids boiling.

Given the two facts above, answer the question "Vapor pressure is caused when" with the following options: 
- liquids expand
 - It gets heated up
 - liquids condense
 - liquids are heated
 - Evaporation
 - Light passing through water
 - Water expanding
 - liquids freeze


 

    liquids are heated     