Fact 1: Mammals generate heat mainly by keeping their metabolic rate high.

Fact 2: And resting metabolic rate is a big contributor to calories burning.

Given the two facts above, answer the question "Mammals generate heat mainly by what?" with the following options: 
- hugging mammals
 - calorie counting
 - with their fur
 - energy of moving objects
 - burning calories
 - sweating
 - selling fur coats
 - flow of electrons


 

     burning calories    