According to a food scientist, lemons and limes are the same thing. In fact, in South America, the color of the thing is a mixture of green and yellow and is know as a limon. In Egypt, if you ask for a lemoon, they mean the green one but if you show up with a yellow one, that's ok, too.
So why are there green ones and yellow ones? Do both exist only in the USA? Is it to satisfy the American market for oddity?