Between the Buttons Puns

I'm home for the holidays so I was lying down in my room when suddenly my brother and father burst in! My brother is holding a tape measure.

They say "Hey, check out this new physics we've invented!" while my brother fiddles with the tape measure.

I looked up and with a mixture of horror and resignation I asked "What?"

"It's the principle of BELLYTIVITY!" while stretching the tape measure between their belly buttons.

Cackling they both ran out of the room.

I'm stuck here for five more weeks. I don't think I'm gonna make it.