Why do people wear watches when they cross the desert?

腦筋急轉彎最早起源於古代印度。就是指當思維遇到特殊的阻礙時，要很快的離開習慣的思路，從別的方面來思考問題。現在泛指一些不能用通常的思路來回答的的智力問答題。腦筋急轉彎分類比較廣泛：有益智類，搞笑類，數學類，成人類等。 腦筋急轉彎是種娛樂方式，同時也是一種大眾化的文字遊戲。

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.