Tear of Light

The Tears of Light are drops of a magical substance that appear within  The Legend of Zelda series.

Twilight Princess
In Twilight Princess, the tears are needed to energize the Spirits of Light. In wolf form, Link collects the tears in the Vessel of Light, a magical container to store the tears. They are held by Shadow Insects that can only be detected when Wolf Link activates his canine senses.

Spirit Tracks
In Spirit Tracks, the Tears of Light are used to power up Link's sword, supplying enough energy to land a blow on a Phantom, and allow Zelda to take control of it. Each section of the Tower of Spirits features three Tears of Light that must be collected to power up Link's sword. Once all three Tears have been obtained, Link can strike a Phantom in the back, stunning it so Zelda can possess the creature. After Link obtains the Lokomo Sword, he no longer needs to use the Tears of Light -- the sword itself is infused with divine energy.