There are two reasons why teeth darken over time. The first reason is the most obvious. Teeth darken because the things we eat and drink will cause staining on the enamel surface and build up over time making teeth appear darker. This type of darkening is easily removed with a good professional teeth cleaning and some simple teeth whitening because it is just on the surface. The second reason why teeth darken is the main reason. Teeth naturally darken over our lifetime as we age. The inner portion of a tooth, called dentin, naturally turns a dark yellow color with age and shines through the enamel and so it makes a tooth look more dark yellow. This type of darkening can be reversed, to a certain extent, with teeth whitening but using a more concentrated and longer wear time whitening gel. The whitening gel must "soak" through the enamel and get into the dentin in order to lighten it. That is why it takes longer and needs a stronger gel.