The answer to this depends on which product you use. One type works by irritating the skin. After you apply the product to you lips, it irritates the outermost layer of skin with a specific ingredient -- most often cinnamon, peppermint or capsaicin (it's what makes peppers spicy). Within a few seconds, more blood flows to your lips, making them appear fuller and darker. Other lip plumpers work by adding moisture to your lips, simultaneously making them appear fuller and smoother.