As well as being chemical-producing glands, testicles are gonads. (This makes them part of the body's "reproductive system".) Apart from "hormones", the other important substances made by the testicles are spermatozoa, which are generally just called sperm. (The word "sperm" is plural, so you do not add an "s" and say "sperms".) The "sperm" are tiny living cells which can join with another cell, called an "ovum" (or egg) inside a female, to start a new human life. Scientists call making sperm "spermatogenesis". Boys begin making sperm when they start growing into men, at a time of life called puberty. 