but you don't know when it will come in handy.

但是说不定什么时候你就用上了呢
If you're a doctor

如果你是个医生
and you've got a patient who's running away from you at the speed of light,

你的一个病人以光速逃跑了
you'll know what to do.

你就知道该怎么办了
Or, if you're a pediatrician

或者你是个儿科医生
with a really small patient who will not sit still,

而你的小病人却总也坐不住
it's because the laws of quantum mechanics

这是因为量子力学
don't allow an object to have a definite position and momentum.

不允许物体同时具有确定的位置和动量
So these are all things

这些就是你们所不了解的
you just don't know when they will come in handy,

却又不知以后哪天能用上的知识
and I teach them because

我教这些内容是因为
these are the things that turn me on

它们使我迷上了物理