"Uh...Mario! Good fight and all that! I must warn you, however... If you happen to find a strange switch somewhere, you should definitely not press it. It's dangerous. Understand?"
Goomba King

Blue Switches, or ! Switches, are items in Paper Mario. They are used to open up hidden parts of the current location Mario is in.

Mario first encounters one of these switches after he defeats the Goomba King and the Red Goomba and Blue Goomba. The switch is located to the side of the Goomba King's Fortress and is hidden in the bush to the side. This switch is used to open the bridge that was previously in place between Goomba Road and Toad Town.

In addition to Blue Switches, Red Switches appear. These switches are activated to solve more intricately designed puzzles.