Fact 1: Thick fur can be used for keeping warm.

Fact 2: Mammals have warm blood, and fur or hair, and breathe air.

Given the two facts above, answer the question "what do mammals keep warm with?" with the following options: 
- fats
 - ears
 - skulls
 - Limbs
 - food
 - tails
 - tissue
 - hair


 

        hair 