咖啡因

kā fēi yīn huì ràng rén shàng yǐn

咖啡因会让人上瘾。

Caffeine can make people addictive to it.

zhòng suǒ zhōu zhī kā fēi yīn shì xīng fèn jì

众所周知，咖啡因是兴奋剂。

It is a well-known fact that caffeine is a stimulant.