From the Super Mario Wiki
When pressed, a P-Switch makes a change in the stage for a limited amount of time. This includes:
Sometimes, a Brick Block with an item inside resembles a coin, but acts as a block. This is fixed in the Game Boy Advance remake of Super Mario Bros. 3.
In Super Mario Bros. 3, a P-Switch can cause the animations of Question Blocks, Brick Blocks and Note Blocks to stop. In addition, reversed coins and bricks don't have any animation (except in Super Mario Advance 4: Super Mario Bros. 3).
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 an extra life. Collecting one more gives Mario two extra lives. After nine coins, Mario will earn three extra lives for each additional coin. Also, gray P-Switches can turn Munchers and Jelectros into regular coins, however, there was no original level that featured both Jelectros and a P-Switch at the same time. Super Mario World is the only game in which P-Switches can be carried by Mario and saved for later. (Mario can also carry the switch after it has been pressed for a short time if he's quick before it disappears.) In all other games, Mario can't carry the switches and must use them right where they appear.
In Mario & Wario, Switch Blocks do not have their "P", but they switch solid and non-solid colors.
In both New Super Mario Bros. and New Super Mario Bros. Wii, some P-Switches could be reused. This is the first instance of P-Switches exhibiting this characteristic. Prior to this, all P-Switches became unusable indefinitely after one activation.
P-Switches appear in Super Mario 3D Land. Here, they are flatter with a light blue "P" printed. When pressed, they become red.
P-Switches also appear in New Super Mario Bros. 2 and New Super Mario Bros. U (including its expansion pack New Super Luigi U), where they serve the exact same functions as in the previous New Super Mario Bros. games.
P-Switches appear in Super Mario 3D World they have the same appearance as they do in Super Mario 3D Land.
A P-Switch makes its first appearance outside the Super Mario series in the Golden Plains stage in Super Smash Bros. for Nintendo 3DS. If a fighter steps on one, Blue Coins will temporarily appear among the ordinary Coins while the P-Switch music plays over the stage music, as in the New Super Mario Bros. series. A P-Switch also appears as a Trophy.
P-Switches are set to reappear in Captain Toad: Treasure Tracker.
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