在床上是用 in the bed 还是 on the bed?

艾老师,你们 sitting in the bed 好玩吗?

啊,这就不对了。

We're sitting on the bed.

那什么时候才能用 in the bed 呢?

我给你展示一下,来。

A few moments later.

And this is in the bed.

你学废了吗?