We've been cataloging plant and animal species in our backyard for biology studies and discovered that wood sorrel has a sour, lemony taste to the leaves because of oxalic acid. This acid is also present in many other fruits and vegetables; in fact, according to Wikipedia, if you eat a slice of rhubarb pie with a glass of milk, it will cause some of the calcium to precipitate out, creating tiny grains you can feel on your tongue!