Q

When do people have two mouths?
什么时候人有两张嘴？

A

When there are two of them.
有两个人时！

点睛一笔

people是“人，人类”的意思，它的单数和复数是同一形式，都是people。所以问题“When do people have two mouths？”并没有限定一个人，所以，有两个人时当然就有两张嘴啦！