Masters and Johnson observed that the foreskin unrolled with intercourse. However, they overlooked a prior observation that intromission was thereby made easier. To evaluate this observation an artificial introitus was mounted on a scales. Repeated measurements showed a 10-fold reduction of force on entry with an initially unretracted foreskin as compared to entry with a retracted foreskin. For the foreskin to reduce the force required it must cover most of the glans when the penis is erect.