From the Super Mario Wiki
The Stop Watch is an item that freezes enemies and/or the environment. It appears in several games. For information about other types of Timers, see here.
Super Mario Bros. 2/Super Mario All-Stars/Super Mario All-Stars + Super Mario World/Super Mario Advance
If the player first pulls up four large vegetables, pulling up what would normally result in a plant will instead reveal a Stop Watch. It will appear for a second and then freeze everything in the level for a few seconds. This gives a window of opportunity for the player to defeat enemies and collect small hearts. However, touching the frozen enemies still damages the player.
Stop Watches in this game cannot be used during gameplay. They act primarily as collectibles which count toward a final game score of 103% and also act to unlock various time trial challenges after King K. Rool is defeated for the 2nd and final time. These time trials are merely select levels from the game which are timed unlike in regular gameplay. Finishing 6 of the 12 total challenges on top of collecting all major items in the regular levels will lead to a perfect game score.
When the player uses it, it will immobilize enemies for a few turns (assuming it is successful). It can be bought at most stores and is a fairly common item.
Here, it is a somewhat rare item, for it can only be purchased in Twilight Town or Fahr Outpost. The item will immobilize enemies just as it does in the game's predecessor. However, enemies can also use the item against Mario or his partners. Several warped clocks appearing in various colors will appear on-screen after using the item; after about four seconds, the item takes effect and immobilizes its prey.
The Stop Watch appears as an uncommon item. It immobilizes all enemies.
In the Nintendo Comics System story Cloud Nine, it appears as a living clock that serves as the Mushroom King's alarm clock, dodging the grumpy King's attempts to swat it and fearfully running to Princess Toadstool for safety.
Names in other languages