Nectar fell down on the earth from their mouths in drops which later grew as garlic; that is why the vegetable has such wonderful medicinal properties.

http://en.wikipedia.org/wiki/Garlic

A belief among some Hindus is that when Devas and Asuras fought for nectar during churning of the ocean of milk (Samudramathan) in the other world, two Asuras were able to get access to nectar and had some quantity in their mouths in stealthy ways. Knowing the Asuras' foul play, God cuffed the heads of those Asuras before they could swallow it and as a result 

nectar fell down on the earth from their mouths in drops which later grew as garlic; that is why the vegetable has such wonderful medicinal properties.