From the Super Mario Wiki
The Super Acorn is an item in New Super Mario Bros. U. The item resembles an acorn with red and yellow stripes on top and a red face with eyes on the bottom. It transforms anybody who touches it into the flying squirrel form, gaining puffy overalls, a grayish shirt and shoes, and a large cap featuring two squirrel ears and a beige brim. Characters in this form also receive a tail and two gray membranes underneath their arms. They also gain the ability to lightly glide in a similar way to the raccoon, caped or bunny forms as well as the ability to cling to walls and air jump while gliding. This item seems to grow from a large Acorn Tree in Acorn Plains.
There are some Super Acorns rolling around in some parts of the world map, where they can be collected instantly and stored into the inventory.
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