You are presented with the quiz "What can contract a little bit all at once as a short term adjustment to the cold?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "shivering occurs when many muscles contract a little bit all at once" and "almost all birds use shivering as a short term adjustment to the cold". 

So, what's the best answer to the question?


 

  muscles         