Self tanning ingredients themselves are actually pretty natural. They are either canola oil, glycerine or sugar based ingredients that combine with the amino acids which are part of the keratin on the skin. Various amino acids combine differently, producing different tones of coloration from yellow to brown. The ingredients can take a day or two before they turn brown. So very simply, the self tanning ingredients make a tan coloured pigment on the skin that looks like a tan, but is not a genuine skin tan which happens when our skin is exposed to UV. Because the colour is made by using the outer layers of skin, the colour will gradually disappear over a period of time as the skin sloughs off. So to keep the colour, it should be re-applied every now and then.