What would happen if you put a pickle in the microwave?

You'd have a big mess. It would most likely explode unless you pricked it with a fork a couple of times before turning on the microwave.

The microwave creates heat by agitating the molecular bond between hydrogen and oxygen atoms in a molecule of water. Anytime you put an object with a lot of water and an outer skin, like a pickle (which is a cucumber), a grape, or an egg, in a microwave and heat it, you create a lot of steam inside the object, and no easy way for the steam to be released. Hence, a likely explosion. That's one of the reasons why you are supposed to poke a few holes in a potato before you microwave it.