"During intercourse, the female orgasm can be elusive. What frustrated woman hasn't wondered: Am I simply, um, put together differently than other women? Kim Wallen, professor of psychology and behavioral neuroendocrinology at Emory University, is busy doing the math to find out. And, yes, he says, simple physiology may have a lot to do with orgasm ease -- specifically, how far a woman's clitoris lies from her vagina. That number might predict how easily a woman can experience orgasms from penile stimulation alone -- without help from fingers, toys or tongue -- during sexual intercourse. In fact, there's even an easy "rule of thumb," Wallen says: Clitoris-vagina distances less than 2.5 cm -- that's roughly from the tip of your thumb to your first knuckle -- tend to yield reliable orgasms during sex. More than a thumb's length? Regular intercourse alone typically might not do the trick."