Common jokes include substituting salt for sugar in the sugar bowl, gluing a coin to the sidewalk, resetting someone's clock, painting their face while they sleep, or placing fake dog doo in a refrigerator. Strikingly realistic dog doo is always a big seller at the Madrid Christmas Market in the Main Square.
But, since the Middle Ages, the best practical joke is to stick a doll on someone's back, formerly made of rags, these days of paper. It's called a monigote and signifies that the wearer is an easily manipulated fool. Last year someone put a doll on a major government spokesman while he was making a speech. One of the free local newspapers comes with a paper doll in today's edition.
So watch your back.