Off the top of my head

Read the following dialogue
and try to guess the meaning of the idiom in bold:

Joe: “ Hey, Mary. Do you
know your sister’s phone number?”
Mary: “ Not off the top
of my head. I have to check my phone.”
Joe: “Would you mind
checking? I really need to talk to her!”
Mary: “Is everything ok?”
Joe: “ Well, I am in love
with her and need to tell her…”
Mary: “ Joe, how many times
have you met her?!”
Joe: “ Off the top of my
head, I guess once or twice.”
Mary: “And you love her
already? How about me? We’ve known each other for at least 7seven years!”
Joe: “ But, Mary… You are so
different from her.”
Mary: “ What? What are you
talking about, Joe? She’s my twin sister!”

Meaning:

Off the
top of my head: to say something without thinking about it
for very long.