A: "What's the purpose of the propeller?"

B: "To keep the pilot cool. If you don't think so, just stop it and watch him sweat!"

A：飞机的螺旋桨有什么用？

B：为了保持飞行员凉爽。不信的话，只要停止螺旋桨转动看看飞行员会不会满头大汗。