From the Super Mario Wiki
This article is about the item that freezes enemies and/or the environment. For information about other types of Timers, see here
A Stop Watch from Super Mario Bros. 2.
A Stop Watch from Paper Mario.
If Mario or his friends first pulls up four large vegetables in Super Mario Bros. 2, 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 Mario and his friends to defeat enemies and collect small hearts. However, touching the frozen enemies still hurts the player, but they will stay in one position and not move at all.
The Stop Watch later appears as an item in Paper Mario. When Mario or his partners use 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.
It appears in Paper Mario: The Thousand-Year Door. 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 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 also appears as an uncommon item in Super Paper Mario, where it immobilizes all enemies.
A related item appears in Donkey Kong Land III. Stop Watches in this game, however, cannot be used during gameplay. They act primarily as collectibles which count toward a final game score of 100%, but also act to unlock various time trial challenges after King K. Rool is defeated for the second and final time. These time trials are merely select levels from the game which are timed, unlike in regular gameplay. Finishing six of the twelve total challenges, on top of collecting all major items in the regular levels, will lead to a perfect game score.
In the Nintendo Comics System story Cloud Nine, Stopwatch appears as an animate object who acts as King Toadstool's alarm clock, dodging the grumpy King's attempts to swat him and fearfully running to Princess Toadstool for safety.
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