Here's a delightful practical joke someone once made for our office Christmas party:

A gelatin mold is made with Knox Unflavored Gelatin and red food coloring. No sugar or flavoring is added.

One would think that a flavorless food would not be at all difficult to swallow, but believe me, from the looks of people who inserted cold masses of this gelatinous glop into a mouth that was expecting sweets, the experience is unexplainably horrifying! Some claimed to be nauseated by the feel of it; others politely swallowed.