From the Super Mario Wiki
Number Platforms (also known as Countdown Platforms) are platforms that appear in the Yoshi's Island series, specifically Super Mario World 2: Yoshi's Island, Yoshi's Island: Super Mario Advance 3, Yoshi's Island DS, Yoshi's New Island, and Yoshi's Woolly World. They appear as a balance with a number labeled on them, with the number shown decreases by one every time something moves on them. When the number decreases to where zero would be, the entire platform instead vanishes in a puff of smoke. They typically range from a highest display of four to a lowest of one, but in Yoshi's Woolly World, some go as high as nine. These platforms don't countdown if the player has Yoshi still on them; they only decrease if Yoshi steps off or jumps off them. They also can only be activated by Yoshi; every other object passes through them.
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