White teeth signal many positive things: good looks, good health, a clean mouth, and a promise of fresh breath, stained or yellowish teeth indicate someone who—true or not—didn’t practice proper dental hygiene. People who are genetically predisposed to having a thin tooth enamel are especially prone to having off-white teeth, because that allows the yellow dentine beneath the enamel to show through. Such people could take excellent care of their teeth, but not have a glowing white smile to show for it. That’s one reason why millions of dollars a year are spent on teeth whitening products and practices. While such an endeavor used to require a trip to the dentist, the advent of self-applied products allows people to whiten their teeth at home.