Knives can become dull over time, after frequent use. Use a knife sharpener or a sharpening steel to sharpen a dull blade. In case you are using a knife sharpener, simply put the knife’s blade in the slot and gently pull it through a couple of times.
If you prefer using a sharpening steel, make sure it has a higher hardness factor than the knife.
Please note that serrated blades cannot be sharpened.
