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 can extend the health meter while small hearts cannot. Collecting a Mushroom also replenishes all empty slots in the meter.
In the remake, Super Mario Advance, they are much more common, being larger and appearing when a thrown object takes out two enemies, and once more for every extra enemy after the second. They also appear from enemies defeated by POW Blocks or Red Shells, and from giant Shy Guys and Ninjis when they are thrown. They are also found in boss areas high in the air, 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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