First, you decide how you toss the Z-Bomb, and then you say where you go to hide. Then the next person does the same, etc.
Examples:
Poster 1: I toss the Z-Bomb at the next poster and run in a cave.
Poster 2: Hits the Z-Bomb with a baseball bat at the next poster and run to a place guarded by Chuck Norrises and a nuke.
etc.

Lets start!

I put the Z-Bomb in an RPG and launch it at the next poster.

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*hides in Godric's Hollow*

I caught the Z-Bomb took a bite of it and gave it to a Patpong performer and she shot it to the next poster.

I caught the Z-Bomb and gave it to a pirate and he shot it to the next poster.

Z-Bomb misfires into my cannon and I launch it back at the next poster.

The Z-bomb lands in a super durable trampoline and bounces to the next poster.

My dragon swallows it and rocket-poops *ewww* it at the next poster.

I catch it and hit it with a tennis racket and hide in a hobo's box