From the Super Mario Wiki
The Heart is an item in the Mario series. It was first introduced in the game Mario Kart: Double Dash!! as the special item of Princess Peach and Princess Daisy. When used, two small pink hearts will appear and circle around the kart. These hearts will protect the kart from up to two obstacles or weapons on the course.
The racer can then use the new item as he or she pleases. However, Spiny Shells, Chain Chomps, and Bob-omb explosions will penetrate the protective barrier. Thunderbolt attacks and falling off the course destroys both circling hearts. Also, any fireballs received do not multiply by five when thrown. If a green fireball hits a heart, the icon for the green fireball received is a red fireball.
The Heart also appears in Paper Mario and Paper Mario: The Thousand-Year Door. After defeating enemies in battle, Mario and his partners may find Hearts left behind in the overworld. Each Heart found will restore 1 HP to Mario and whichever partner is active at the time.
The Hearts also appear in the Modern Game & Watch Gallery series. They can usually be obtained by getting 200, 500, and 700 score points, but they'll only appear if the player has one or two misses. Obtaining it will remove a miss from the screen.
Hearts reappear in Yoshi Topsy-Turvy. They are small hearts that restore one point of Yoshi's health meter when swallowed. There is a rare variation of these hearts called Big Heart, which restored all of the health back.
Hearts also appear in Paper Mario: Sticker Star, where they are made of cardboard and replenish 5 HP of Mario's health. They can be found in many places, such as behind bushes.
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