It's pretty simple. Basically, the first person comes along and inserts anything they want into the vending machine. Then, the second person comes along and posts what the first person received from the vending machine, followed by what they wish to insert, and so on! Like this:
Screename1: I insert a poodle into the vending machine.
Screename2: Screename1 receives a cucumber.
I insert a accordion into the vending machine.
Understand? The point of this whole game is to make what you put in and what the person receives as random as possible. Go nuts! Just make sure it's vaguely Al related so we have a reason for it to be here.

I insert a dozen, starving crazed weasels into the vending machine.