After a few weeks of use, new knives will become dull, forcing you to apply more pressure to make cuts. That added resistance doesn't just mangle meats and veggies; it's a potential hazard for your fingers.
Disclaimer time: I'm stating the obvious, but knives can hurt you. Even if you don't plan on servicing your own blades, handle them with care. The smallest of paring knives can cause a big injury in a flash. Always exercise extreme caution and be mindful when using these sharp implements -- for your sake and of those around you. I'll also focus on steel knives since ceramic blades typically require professional servicing.
When your knife’s blade has taken a beating and needs more help, it’s time to step things up and get out the sharpener, which should only be used a handful of times a year.
Electric sharpeners may seem like the superior choice, but they’re probably the worst tool for sharpening a knife. I learned this in culinary school, when a fellow student pulled one from his tool kit and our French chef instructor swiftly swatted it across the kitchen. Electric sharpeners strip too much metal from the knife, destroying the blade and weakening it over time. (Which is a shame if you spent a lot of money on your beloved knives.)
Handheld sharpeners are OK, but they’re not the best.
What you really want is a whetstone (also known as a sharpening stone or water stone), which is the preferred tool for sharpening knives because it gives you, the cook, complete control. Sharpening stones are basically long, rectangular blocks of composite stone, typically with a coarse grit on one side and a fine grit on the other. As seen below, the side with the lower number is used for sharpening, and the side with the higher number is used for polishing:
How to sharpen your knives using a whetstone
Start by soaking the stone for 10 minutes to 20 minutes in water, which will help lubricate the knife as you sharpen. Place the stone on a towel or mat with the coarse side up. Next, a very important step: Establish the knife’s angle, which is usually between 20 degrees and 25 degrees for European-style knives, and as steep as 15 degrees for Japanese-style knives. Hold the knife firmly at the appropriate angle and slide it across the stone, pulling it toward you, starting from the heel and working toward the tip. Do about 10 to 20 strokes per side, then repeat on the fine grit side. Wipe the knife clean and it’s ready to use. Test it on a tomato to see how sharp it is, and you’ll be amazed.