A Red Apple is an item in Super Paper Mario. Found on a tree in Overthere Stair, it restores five HP and cures poison. If the player tries feeding Peach this apple, she wakes up from the deep sleep caused by the Golden Apple and wonders what's happening. Then, suddenly, she grows huge, about the size of the tree. A few seconds later the effect wears off, Peach shrinks, and goes back to sleep.
This is a clue for later in the game, when the player has to feed a cloud named Cyrrus a Red Apple to make him grow huge. This makes the player able to bounce on him.