Larks' Water is a kind of water that, once drunk from, causes a boy to pounce on any girl happily.
Evenaugh Valiant says that once a boy drinks from it, he happily pounces on even the ugliest girl, but that it doesn't really work on Dwarfs. Some say that it wears off after one attack, but others believe it lasts for months. The Witches believe it only brings out what's already in your heart. Larks' Water doesn't keep, and that the effect is too unpredictable.
It's possible that when two larks die on a stream, once the water from that stream is drunk from, the subject jumps on any girl. It is unknown whether it is the case with females, though that is very likely, because Clara doesn't resist when Jacob kisses her.
Larks' Water is cool, and Jacob comments it tastes earthy and fresh.