The poster posts what to insert into the vending machine. The next poster posts what they got out of the machine, then what they want to insert into the machine.
Example:
Poster 1 (me) - :inserts coin: _____________
Poster 2 - You get a computer keyboard! :inserts CD: _____________
Poster 3 - A kitty cat comes out to lick your face!! :inserts a piece of tape:
And on and on, get it!? good! I will start first...