If you have ever had the chance to go skinny-dipping in cold water with a man, you may have noticed that his testicles do raise and lower. When they come into contact with cold water (or any cold temperature) they snug up to the base of the penis. The testicles hang in a sack of fleshy skin called the scrotum. They are outside the body, but inside the scrotum, because the temperature of the core of the body is too warm for them. The testicles have two main functions to perform: manufacturing sperm and manufacturing testosterone. This outside-the-body location creates the optimal temperature for testicular functioning - about 94 degrees F. Therefore, one of the main reasons you might see testicles moving around relates to temperature. The cremaster muscle is responsible for the movement. It expands and contracts according to temperature - moving them further from the body when it's warm and closer to the body when it's cold. (In really cold lakes, they can't seem to get close enough!) Another reason for testicle movement does have to do with sexual arousal. In the 1960's, Masters and Johnson observed that the testicles elevate just before ejaculation and actually make direct contact with the body. They found that in about 85% of men the right testicle rises before the left one. Masters and Johnson stated that this movement is very important and that at least partial elevation must occur in order to have what they called a full ejaculation. Sometimes older men will have varying testicular movement and report that there is less pleasurable pressure associated with their ejaculation when their testicles elevate only slightly. Younger men can also have the experience of less pressure when they ejaculate involuntarily and before arousal has reached its peak. During arousal the testicles also increase in size due to vasocongestion, the accumulation of blood in the pelvis that occurs during arousal. The skin of the scrotum thickens and the testicles increase in size usually by approximately 50% at the height of arousal. However, Masters and Johnson found that if sexual excitement is sustained long enough, the testicles could almost double in size, returning to normal size after orgasm. They also found that this increase in size tends to taper off as men age.