Fact 1: Eyes are used for seeing by animals by sensing light.

Fact 2: Infrared light is invisible to the eye.

Given the two facts above, answer the question "what light can animals not see?" with the following options: 
- Sand dollars
 - electricity
 - motion
 - sun's heat
 - photons
 - our star
 - heat
 - infrared


 

        infrared 