Fact 1: Mammals can also generate little bursts of heat by shivering.

Fact 2: Shivering causes muscle contractions to warm the body.

Given the two facts above, answer the question "Mammals can also generate little bursts of heat by" with the following options: 
- muscle contractions
 - circulation
 - exhalation
 - Temperature changing
 - Movement of an air mass
 - flow of electrons
 - vibrations
 - with their fur


 

 muscle contractions        