From the Super Mario Wiki
A Small Heart is an item found in Super Mario Bros. 2. It would fly out every time eight enemies are killed. When Mario, Luigi, Princess Peach or Toad grabs it, they replenish an empty slot in their Health Meter. They have somewhat the same function as a Mushroom in this game, with two exceptions. Mushrooms both extend and replenish the health meter, while small hearts cannot.
In the remake, Super Mario Advance, they are much more common (even appearing in boss arenas), being larger and appearing when a thrown object takes out two enemies in a row. They also appear from enemies defeated by Red Shells, and from giant Shy Guys and Ninjis when they are thrown or beaten. They are also found floating in midair, and in plants, referred to as Heart Radishes.
It also appears in Dance Dance Revolution: Mario Mix. When used, it will refill half of Mario or Luigi's dance meter if it empties. It can be bought at any shop in Story Mode for seventy coins.
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