Take a look at your keyboard. Notice how the M and the N key are just sitting there side by side? It's easy to confuse them. Especially if you swap around the keys on the keyboard.
Pry off the keys using a screwdriver or a house key. They should pop off rather easily. Swap the keys and press them back into place.
The great thing about this prank is that it can be done with just about any keyboard you run across.