Personally, I find nothing more frustrating in the kitchen than a dull knife. Not only does it make prep work a chore and your finished product less attractive, it's also downright dangerous. A dull blade requires more pressure to cut into a food, and can easily slip off of a tough onion skin and into your finger. Ouch. Here's a slideshow on how to sharpen a knife with a sharpening stone, with recommendations on what stones to buy.