A: When is your mother in law's birthday?
B: I don't know off the top of my __________. I'll check it in my diary.

"I don't know offhand" means that you do not know it without having to ask someone else or look it up in a book.

I haven't a/an _________ where I put my keys! I can't find them anywhere.

 to learn something in order to be able to repeat it from memory, rather than in order to understand it

to learn something through experience, usually because you've made a lot of mistakes

to learn something because you had a tutor or teacher explain it to you

I know the lyrics to my favourite song by __________.

"I haven't the ___________ idea about computers. I always phone my brother when there's a problem with my laptop."

We say 'to know something inside out' to describe a ________ that you know very well.

A: When is your mother in law's birthday? B: I don't know off the top of my __________. I'll check it in my diary.