Your knife is too dull. How do I know this? For one, you clicked on this article. For another, you're supposed to be sharpening (or at least honing) your knife every few weeks—and, well, most of us don't do that.
Don't risk cutting off a finger tip while you're cooking Christmas dinner (or accidentally slicing your palm open as you chop potatoes for latkes). Learn how to sharpen your knives instead.
A dull knife has ridges and nicks in it that are invisible to the naked eye (unless you've really, really damaged your blade). If you had a high-powered microscope, though, you could see how smooth and even a knife is after it's sharpened. A dull knife, on the other hand, looks like a piece of tissue paper that's been haphazardly torn. You can see for yourself—Cook's Illustrated took awesome side-by-side photos with one of MIT's high-powered microscopes. That smooth blade will cut through almost anything like it's warm butter.
Getting your knives into tip-top shape is pretty easy. Step one is to hone them. Go grab that long steel rod that came with your knife set (but now just sits in the block because you never learned how to use it). It's called a sharpening steel. Hold it in one hand and take your knife in the other. Now listen to this lovely British man explain how to slide the knife along the steel properly:
Technically, what he's doing isn't sharpening the knife—it's honing it. That basically means that he's re-aligning the edge of the blade so that it forms a smoother, cleaner line. It will make your knives feel sharper, and they'll cut through food better. Plus, it's easy to do. So make it a habit to hone your knives every few times you use them.
To truly sharpen your knife, you have to remove a tiny bit of the metal and reshape the blade. If you want to impress your friends, you can use a traditional whetstone. All you have to do is place the knife at a 20-degree angle to the stone and slide the entire length along it with light but firm pressure. Make sure to alternate sides with every stroke so the blade stays centered. Start with the coarse side and move to the finer side. Bonus: it makes a satisfying scraping sound that will make you feel like a knife pro.
Whetstones are cheaper than most other knife-sharpening options—and they're more versatile—but you have to commit to learning the method. Whatever angle you hold the knife at while sharpening will become the angle of your blade, so if you do it wrong you'll end up with a less-than-ideal cutting implement. The more acute the angle, the sharper the blade. You don't want to be obtuse about knife sharpening techniques.
You can also use an electric or manual knife sharpener if you're less confident in your ability. You just won't feel like as much of a bad-ass.