He was also puzzled by the black stones people used to burn to produce heat.

他也对人们用来燃烧产生热量的黑色石头感到困惑。

You see, anything that uses a lot of energy is going to produce heat, and we don't observe this.

任何使用大量能量的东西将会产生热量，但是我们没有观测到。

When two objects are rubbed together they will produce heat.

两个物体摩擦会生热.