Even if you adhere to near-perfect dental hygiene, you can still end up with yellow teeth. Your age, the foods you eat and your lifestyle habits can all contribute to stains and yellowed teeth that aren't exactly picture-perfect. Of course, thanks to the availability of whitening toothpaste, you don't necessarily have to see your dentist to restore your gleaming white smile. But before you pick up a toothpaste that promises to whiten your teeth, make sure you understand the hows and whys so you can pick out the right product.