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Wurm's Tooth

Whenever a player casts a green spell, you may gain 1 life.

Jevna Nor lugged it around, bragging of his triumph over the wurm. Luckily, the tavern regulars didn't notice it was only a baby tooth.

Magic 2012

Salvat 2011

"The great wurm bit down on me. Instead of being crunched bloody, I slid into the space where its tooth was missing. One tooth—a boon most potent, indeed."
—*The Tall Tales of Oleander*

Magic 2011

A wurm knows nothing of deception. If it opens its mouth, it plans to eat you.

Duels of the Planeswalkers

Magic 2010

If a player casts a green spell, Wurm’s Tooth’s ability triggers and is put on the stack on top of that spell. Wurm’s Tooth’s ability will resolve (causing you to gain 1 life) before the spell does.

The ability triggers whenever any player, not just you, casts a green spell.