P Switch

P-Switches (also known as P-Switch Blocks in Super Mario World, New Super Mario Bros., and New Super Mario Bros. Wii) are items found in Super Mario Bros. 3, Super Mario World, New Super Mario Bros., New Super Mario Bros. Wii. When pressed, a P-Switch changes something in the stage for a limited amount of time. This includes: One rather confusing thing is that a brick block with an item inside it will look like a coin, but act like a block. This confusion was removed in the Game Boy Advance remake of Super Mario Bros. 3.
 * Turning Brick Blocks (Used Blocks in Super Mario World) into coins.
 * Turning coins into Brick Blocks (Used Blocks in Super Mario World).
 * Changes enemies and Munchers into Silver Coins (Super Mario World gray switches only).
 * Allowing Mario to access a hidden Door.
 * Makes Blue Coins appear.
 * Makes hidden Question Blocks appear; Ex.: In the level, Gnarly.

In Super Mario World, the gray version of the P-Switch makes most enemies turn into Silver Coins. Collecting eight of these coins awards Mario a 1-Up. Collect one more for a 2-Up. After nine coins, Mario will earn 3-Ups for each additional coin.

Trivia
P-Switches are seen in Super Mario 3D, and these are used with the same purposes. This switch has a flattened shape with a P in the top face.