Fly peeing

A little old lady is walking down the street, dragging two plastic garbage bags, one in each hand. There's a hole in one of the bags, and once in a while a $20 bill flies out of it onto the pavement. Noticing this, a policeman stops her.
'Ma'am, there are $20 bills falling out of that bag.
'Damn!' says the little old lady .'I'd better go back and see if I can find some of them. Thanks for the warning!'
'Well, now, not so fast,' says the cop. 'How did you get all that money? Did you steal it?
'Oh, no,' says the little old lady. 'You see, my back yard backs up to the parking lot of the football stadium. Each time there's a game, a lot of fans come and pee in the bushes, right into my flower beds! So, I go and stand behind the bushes with a big hedge clipper, and each time someone sticks his thingie through the bushes, I yell:"$20 or off it comes!''
'Hey, not a bad idea!' laughs the cop. 'Good luck! By the way, what's in the other bag?''Well,' says the little old lady, 'not all of them pay.'