Why do penis' get stiff?

The physiological process of penile erection, also known as tumescence, is primarily controlled by the autonomic nervous system. When a man becomes sexually aroused, the brain sends signals to the nerves in the penis, causing the smooth muscles in the corpora cavernosa to relax and allow increased blood flow. This influx of blood fills the spongy tissue in the penis, leading to its engorgement and stiffening. Erection is a complex interplay of vascular, hormonal, and neurological factors that ultimately enable sexual intercourse and reproduction.