This is simple. You think of a design or pattern for a magical mirror that will turn you into whatever the next user posts.
If that's not enough, here's an example:
User1: My Mirror is white with magnets.
User2: You turn into a fridge. My Mirror is polka-dotted.
They can also be funny:
User1: My mirror says "Win" on it.
User2: You turn into "Fail".
Simple enough.
I'll start:
My Mirror has wires and is electronic.

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You turn into a Wii.
My mirror has Scratch Cat on it.
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You black out. then you wake up, and feel wierd and orange. You are now a Scratch Cat!
My mirror has a red "M" cap on it.
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You become mario!
My mirror has a ROBLOXian on it. o_O
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You get shot by robloxs
My mirror has a pokemon on it
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You become Pikachu???
My mirror has a legoman in it.
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you turn into legoman.
My mirror has a built in toilet!
BTW you guys, they can be as wierd, funny, or strange as you like.

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You become p00 o_O
My mirror has nothing on it.
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You turn into nothingness. My Mirror has Astro Boy on it.
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It turns you into rocket, and you blast off to the planet of zorbgon, home of the kryptonite monkies.
My mirror has a beautiful princess on it, and she gives you three wishes!
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The fish are pirhanas, and you turn into bait.
My mirror is made of chocolate and is surrounded with junk food.

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