Why is testicle hard?

The testicles are built as a temperature control for the sperm. The temperature sperm is mos t comfortable is slightly lower then normal body temperature. So when it is hot the testicles hang low and loose allowing more air around them to cool them. If it is cold the pull in tight to the body in order to keep them warmer. They also pull in closer during sex to keep them out of the way and because of the muscular tension in the area.