Leeks can sometimes have some dirt or grit stuck between it's layers. A leek is like a bunch of tubes layered on top of each other so if you slice them crossways you will get small rings. The grit can hide between those rings. There are a few ways to clean them, but this one works for me: slice out the white section you want to use. Pull off any discolored or dried outer layers and discard. Make a slice lengthwise to cut the leek in half, then repeat that with each half so you end up with 4 long quarters. Hold each quarter under running cold water and manipulate the layers so the water rinses out any grit. Place the cleaned leek on some paper towels to drain.