EXPRESSION: "OFF THE TOP OF MY HEAD"

EXPRESSION: "OFF THE TOP OF MY HEAD"

I say this quite often because when I am in class, I don't know all the answers to the questions that my students ask me, so I say: "Off the top of my head, I am not really sure...but I will get back to you on that!".

"Off the top of my head" basically means that you are saying the first thing that comes to your mind without thinking about it too much.

​What would the Spanish version be?