Leeks are generally quite sandy and need to be washed before using. Trim the root end and the tough, dark-green leaves from the leeks. Then slice the leeks crosswise and place the slices in a big bowl of lukewarm water. Swish them around vigorously in the water and let stand for several minutes; the dirt will settle to the bottom of the bowl. With your fingers, lift the leeks out of the water. If they are particularly dirty, repeat the process, using fresh water.