Where are your bones?

My grandmom had walked more today than she normally does. So later after dinner, my grandad and grandmom were sitting and chilling with me when ...

Grandmom: Oh i'm so tired.
Rusty: Yeah ... you walked a lot today.
Grandad: Its good you're getting out of the house more.
Grandmom: Every bone in my body hurts.
Rusty: So you have bones outside your body too? ....
Grandad: Bones can't burst.
Rusty: ..... what?

Anyway it turned out my grandad misheard. He's a bit hard of hearing. He heard burst instead of hurt. Strange things are happening here.