I always keep the theory behind things simple for clients, so's not to bore them, more than anything else!! ....here's what I say to my clients, if I'm asked :- " the DHA, is the active ingredient in the tan, which reacts with your skin & gives you your colour" DHA (dihydroxyacetone) reacts with the amino acids in the dead layer of your skin. The DHA combines with the amino acids In the skin to form brown-coloured melaninoids. It is not absorbed into the body & is obviously non-toxic. It was discovered back in the 1920's & is derived from cane sugar. Hope this helps x