Most pastes, including Aquafresh Extreme Clean Plus Whitening, contain silica, a gritty mineral that scrubs away surface stains caused by things like coffee and red wine. "Silica is good at removing superficial stains," says Lieb, "but it will not whiten the tooth itself." For that, you need a product like Rembrandt Plus Premium Whitening Toothpaste ($7.50; that has urea peroxide, a proven bleaching agent.