From the Super Mario Wiki
Winged Clouds are clouds with a green question mark on them that use wings to fly. They appear in Super Mario World 2: Yoshi's Island, its Game Boy Advance remake, Yoshi's Island DS, and will appear in the upcoming Yoshi's New Island for the 3DS. and are found in almost every level.
When a Yoshi hits a Winged Cloud with a Yoshi Egg, it disappears and reveals its contents or triggers an effect. Winged Clouds can contain objects such as Flowers, Stars (tip: if you see five stars it will give you five more seconds to increase the Countdown Timer-one from each star), Extra Lives and ! Switches, but sometimes also creates stairs or bridges or makes coins appear instead of revealing an item. Due to their function, they can be considered Yoshi's Island's equivalent to ? Blocks.
The Winged Cloud Maker is an item that can be won in Mini Battles and Bonus Challenges, and it turns every enemy on the screen into a Winged Cloud. These most often contain coins or stars, but can sometimes contain an Extra Life.
Winged Clouds appear in volume 14 of the Super Mario-Kun, also providing items when they are hit, such as a Super Blue Watermelon. Unlike in the games, however, Yoshi does not need to hit them with an egg to make them spawn items.
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