What would you think if you heard this: Off the top of my head.
There is nothing on her head. But let's see if we can find out what she means.
A: Do you know Tom's email address?
B: I couldn't tell you off the top of my head, but I have it in my contacts list on my computer. Can I send it to you later?
A: Sure. That would be great. Thanks.
"Off the top of my head" is a common phrase that means I am trying to remember something, and to give you a quick answer. But if a person's memory is bad, sometimes the information off the top of someone's head is wrong. So it's best to confirm it.
And that's English in a Minute.