The cup has a hole in the bottom (obviously), surrounded by an FDA-approved magnetic o-ring. IN the bottom of the cup is a similar FDA-approved round refrigerator/phone book type magnet that seals itself to the o-ring, except when the beer dispenser is pushing through the 'flap."
Cups
you can also get the round magnet printed with your business name, etc. When a person is done with the cup, they can remove the round magnet and toss it on the fridge, or something.