What else is hollow?

So I said I wouldn't be posting, but I was bored. So here is what I have to say:
I was eating my lunch, and I stuck my fork into my avocado, and I was surprised to find that it was hollow. So I did what anyone would do and said:
"My avocado is hollow."
And my extremely mature sixteen-year-old brother replies by saying:
"You know what else is hollow, Isabella?"
And because I am always a step ahead, I am quick to say:
"Your head." Yes, I know, I am incredible.
And then Nigel (my extremely mature sixteen-year old brother) says in a slightly melancholy voice:
"That's what I was going to say, but about your head."
And because my head is not hollow, and his head is, I am ready to say:
"Nigel, I'm smarter than you. Don't try to out-wit me."
This was for all the younger sisters of extremely mature sixteen-year-old brothers.
~Isabella