Sun-worshippers seeking the ultimate all-over tan should admit defeat, say scientists, who have proof that it is a physical impossibility.
The University of Edinburgh team found different parts of the body are much harder to brown in the sun.
Buttocks are the least easy to tan compared to backs, they told the journal Experimental Dermatology.
Hard-to-tan areas will need more sun protection against cancer, they warn the public.
The scientists carried out their research to find out why skin cancers like melanoma tend to be found in different parts of the body even though they are all caused by exposure to harmful ultraviolet (UVB) sun rays.
They asked 100 volunteers to go under a sun lamp to see how easily they tanned.
After six tanning sessions the scientists analysed the skin of two body areas - the back and buttock.
Using scientific instruments they measured the precise depth of tan achieved.
They found buttocks were particularly resistant to tanning, staying whiter than backs despite the same exposure.
But this does not mean that it is more resilient to the sun's harmful rays, say the researchers.
The tan colour comes from the skin's production of melanin, a defence that blocks the skin from absorbing too much harmful UVB radiation.
Professor Jonathan Rees, who led the research funded by the Medical Research Council, said: "One of the real puzzles about melanoma is why the numbers of tumours differ so much depending on body site.
"Our work shows that in one sense we are all made up of different units of skin, which responds differently to sunshine, and which all may afford different degrees of protection against the harmful effects of sunshine."
Dr Jodie Moffat, health information manager at Cancer Research UK, said: "This study is interesting because it suggests that different parts of the body respond differently to UVB - one of the types of radiation given off by the sun.
"Tanning is a sign that your skin has already been damaged by UV and is your body's way of trying to limit any further damage.
"For people with fair skin, freckles or lots of moles, tanning is very difficult without burning, and they should take extra care to protect their skin from sunburn."