The process of tanning- UVA rays trigger melanocyte cells in your skin to produce melanin (the brown pigment we call a tan) which is the body's method of protecting our skin from burning- so tan comes from underneath the epidermis. Fake tans sit on the epidermis (outer layer of skin) and pretty much dye it, so as the natural process of shedding skin happens, the tans fades/seems to wash off (skin is constantly shedding and washing helps it to come off quicker instead of it clinging onto your body). Hope that answers your Q (no- you'll still get a real tan even with fake on!) :)