When your favorite knives start squishing ripe tomatoes instead of slicing them, it's time to sharpen your blades. This knife sharpener is remarkably easy to use on both straight-edge and serrated knives. Simply plug it in, turn it on, and feed knife blades through the stages. The first sharpening stage uses 100 percent diamond abrasive disks to hone the knife's edge while the second stage incorporates polymer disks to polish and strop the edge to hairsplitting sharpness. Straight-edge knives should go through both stages, while serrated blades should only be used in the polishing stage.