The fourth wall
Because they've no pockets to put things in!
Cash or Czech Edit: a word
Because they have tears in their eyes
Marriage
My Zipper
The Star Spangled Banner!
For those of us that struggle with our family perhaps this will help break the ice.
Ten. (Shame this one doesn't work too well for reddit - the funniest part of this joke is the third punchline enjoying people struggle to understand what the hell you're talking about.)
Darth-tagnan
The Fourth of Ghoul-ly!