I made my sub pee into a condom. As he peed the condom grow and grow and grow with his warm pee. When it was full to bursting he tied a knot in it and we had a little play with the warm rubber sack of warm wee wee. It was a weird sensation actually, like a slinky. After that I popped the condom with a pin, and pee splashed out of it into the bucket we had handy. Good job too.