Off the top of one's head!

Any ideas of this meaning?

This idiom "off the top of one's head" means impromptu, without previous thought.

It is often used after the verbs remember and have

For example: Anna asked me whether I remembered any her friends number. Unfortunately I didn't have any off the top of my head.

Yesterday I went to supermarket but I couldn't pick anything up, because I couldn't remember anything off the top of my head.

Got it?