in指两膝盖之间
on指两膝盖之上
可以通过例句来理解：
Are you referring to your lap as if it was a container of something? In your lap ✔️
你指的是你的膝盖，好像它是一个东西的容器？
Are you referring to your lap as though it was a surface which something was on? On your lap
在你的膝盖上 你是指你的膝盖，好像它是一个表面，上面有什么东西？在你的腿上
