My passport stays in a money belt, always.

Wait, I have to disagree a little! I always heard it's more about ownership than safety. My cat does it most when new stuff comes into the house, like a bag or a person. It seems like he's saying "this is mine now" more than "this is safe." He's not a clingy cat at all, he's pretty independent. The book probably has the right facts, but I see the meaning differently. It feels like a territorial thing first and a comfort thing second.

Yeah @victor_lane60, read that smaller diameter means less flex.