Mechanics/Friction
In order for sexual intercourse to be possible, the rigidity of the penis must be sufficient to overcome the closing pressure of the vagina. The shape of the head of the penis resembles a mushroom cap, which nature seems to think is best for penetrating solid earth to sprout. The penis elongates and enlarges further after vaginal entry and curves upward (which is referred to as 'boomeranging') and at the same time the vagina elongates and the cervix repositions to optimize sperm entry. Personal lubricants of all types facilitate intercourse. Partners applying a lubricant to each other communicate their wanting to give pleasure, and genital touch is the most potent stimulus for arousal. Penile rigidity and vaginal receptivitity are improved, and the decreased friction facilitates entry. Abnormally high and sustained vaginal muscle tone, preventing entry with a normal degree of rigidity is called 'vaginismus'. Foreplay and lubricants are essential in overcoming this involuntary condition.

A few "silicon" lubricants have come onto the market. These products use a low per centage of a liquid silicon compound (dimethicone copolymer), which is used in hair care products, and which dissolves in water. The silicon component does not evaporate, making it long lasting, and it is inert, so that it will not irritate genital skin. It is messy and stains cloth, however the real problem is that it is too slippery. Friction needs to be reduced slightly for entry, but if it is reduced too far, pleasure fails. Orgasm occurs when there is a release of oxytocin in the brain by cells in the hypothalamus. We have been working on the notion that there is a chemical messenger pumped into the blood stream by rhythmic genital touch, intercourse or masturbation that triggers the effects in the brain. A 'good' lubricant is like vaginal fluid.