Why do people wear watches when they cross the desert?

1、What is it that is not burnt in the fire, and can't be drowned in the water?

2、I tell heat, and I tell cold, and they in turn tell me to go up and down as I am told. What am I?

3、Why do people wear watches when they cross the desert?
 

答案：
1、Ice.

2、The mercury in a thermomether.

3、Because watches have springs in them.