i needed to write this down, least i forget.
people debate over whether the ability to roll your tongue is genetic or if it can be learnt.
in a sense it is genetic, it is a GENE that allows it to happen. it is a dominant allele, so if one parent has the gene it will be dominant over the parent that doesn't and it is likely the child will be able to roll their tongue.
the gene itself codes for the production of a protein that affects the muscle in your tongue. the tongue is a muscle, and all people can move it. this is because the muscle is about to contract vertically, to pull your tongue up and backwards.
people-who-can-roll-their-tongue's muscle is also capable of contracting horizontally. this is because they have that gene that codes for that particular protein. it means that their tongue muscle is able to contract horizontally and pull in the sides of their tongue, hence 'rolling' it.
i don't know why i suddenly so desperately needed an answer for this, i'm supposed to be revising chemistry but there you go.
if this is wrong, shoot me. it's the best i can gather from all the different debates/arguments i've read or heard about. and for the record biologists haven't actually agreed on the phenomenon yet. i'll get back to you.