Fact 1: Shivering occurs when many muscles contract a little bit all at once.

Fact 2: Almost all birds use shivering as a short term adjustment to the cold.

Given the two facts above, answer the question "What can contract a little bit all at once as a short term adjustment to the cold?" with the following options: 
- that metal
 - muscles
 - Weight
 - stomach
 - mammals
 - atoms
 - ligaments
 - sweating


 

  muscles       