You put something in the vending machine and then the person below you says what comes out. It can be something related to the item, something that sounds like the item or even something completely random. If you want, just to make it funnier, you even make a pun out of it.
Example:
Person 1:
Inserts lemonade.
Person 2:
Gets urine.
Inserts football.
Person 3:
Gets basketball.
Inserts giant man eating spider.
End of example.
Have fun people.
Inserts knuckle duster.