Heart

Recovery Hearts are omnipresent items in The Legend of Zelda series.

Location and Uses
Hearts aid Link by replenishing his life energy by one heart at a time. They can be found anywhere in Hyrule, Koholint Island, Termina, Labrynna, Holodrum, the Great Sea, the World of the Ocean King, and Skyloft. Recovery Hearts are uncovered in pots, jars, tall grass, defeated enemies, under rocks and, in many instances, they can even be purchased from shops. They appear as small, floating, red hearts.

Life Meter
In various The Legend of Zelda games, Link can extend his life energy from three to twenty hearts. In others, the maximum is 14 or 16. The only game where Link doesn't have his health represented by hearts is The Adventure of Link, where it's represented by rectangular segments (the game still features Heart Containers to increase the life meter, however).

 Phantom Hourglass introduced a Green Recovery Heart. These replenished the endurance of the S. S. Linebeck and were only found while traveling the Great Sea. These same types of Recovery Hearts are used in Spirit Tracks to heal the Spirit Train.

Oxygen
In both Ocarina of Time and Majora's Mask, oxygen is measured according to Link's life meter. When he is exposed for too long underwater, or enters a very hot environment, a timer appears. Before it runs out, the young hero must either rise to surface or exit from the hot area, depending on the area; otherwise, he will die. How long the time will be depends on the current life meter (not only the maximum capacity, but also the current amount of hearts). For every eighth of a heart Link has, he will have one second on the timer. For example, if he enters Death Mountain Crater with a three-heart meter unharmed, then he will have 24 seconds to exit from there. The timer remains unaffected by the Enhanced Defense upgrade.

As a Gesture of Gratitude or Love
One humorous moment in Twilight Princess occurs when Link defeats the boss of the Snowpeak Ruins, Blizzeta, who is actually Yeta, the wife of Yeto. After she is restored, Yeto rushes to Yeta's side and the pair share a romantic moment. At this point, forty recovery hearts burst from the pair, as well as a Heart Container. Similarly in Twilight Princess, after Link beats the STAR Game in Castle Town East, he can gain three hearts by approaching Kili, Hannah and Misha. More hearts also appear near Honey and Darling in Majora's Mask, although Link is unable to collect these Hearts.

Another moment occurs in Ocarina of Time: as Link and Zelda reach specific floors in Ganon's Castle, Zelda becomes trapped in a ring of fire and two Stalfos appear. The only way to free Zelda is to defeat both foes; once this occurs, she thanks Link and drops a few Recovery Hearts.