fake blood.
Or better yet, pour a little coffee onto the bottom. At first glance they will think it's just a full cup of coffee. After the accident, they will look silly holding an empty cup with coffee spilled.
great idea! ventsi you should consider adding this to the prank!
oh i get it now you leave it on thier desk until they come!
Why not color the bottom brown or paint it then they would think coffe was in it.

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