An imperfect method of forcing girls into vague sexual acts. Accusing them of frigidity would generally result in a denial, whereupon you would be perfectly within your rights asking them to prove it. If she agreed then you got to feel her ladybumps while she stood as still as she could. Most girls would usually tell you to piss off though.

You can, according to my better-informed schoolmates, tell whether a girl is a virgin or not by whether her knees rub together when she walks. If they don't then she has undoubtedly been riding the entire town and should be buried in a Y-shaped coffin, the dirty little whore.
Let's take Michelle McManus. Her knees definitely rub together when she walks, because she is fat. As it is a well known fact that it is harder to pull fat, ugly, birds than slim attractive ones*, and because, well, NO-ONE wants to shag a fat lass**, this must be true.
*Unless you are a fat ugly bloke.
**Unless you are a fat ugly bloke.