The testicles are attached to the penis. When the penis is inserted the testicles produce sperm cells which travel down a "cord" and into the penis. The sperm cells then go to the female egg and produce a new baby!
Sperm refers to the male reproductive cells. In mammals, sperm develops in the testicles and is released from the penis. Sperm has a limited life span. Motile sperm cells swim toward the egg for fertilization.
Testes produce sperm cells while the seminal vesicles hold the sperm cells as well as liquids to nourish the sperm cells and keep them mobile. During sex, pressure builds up in the seminal vesicles, and when bands of muscles (sphincters) open, the semen squirts out the penis.