Why do people wear watches when they cross the desert?

Why do people wear watches when they cross the desert?

　　脑筋急转弯最早起源于古代印度。就是指当思维遇到特殊的阻碍时，要很快的离开习惯的思路，从别的方面来思考问题。现在泛指一些不能用通常的思路来回答的的智力问答题。脑筋急转弯分类比较广泛：有益智类，搞笑类，数学类，成人类等。 脑筋急转弯是种娱乐方式，同时也是一种大众化的文字游戏。

　　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.