What makes a man male? -Testosterone! Testosterone is the male sex hormone. In males, testosterone production begins before birth, influencing the development of the male genitals, the brain, kidneys, muscles and liver. In childhood, testosterone levels are low, with production increasing rapidly at puberty. Secreted from the cells of Leydig in the testes, it controls the development of secondary sexual characteristics. In teenage boys testosterone creates a deeper voice, a larger skeleton, body hair, developing reproductive organs and "male" patterns of behaviour.

Testosterone increases muscle bulk as it encourages protein formation and induces the retention of nitrogen. Testosterone controls sperm production, sexual drive and sexual performance throughout a man’s life.

But to define it purely in terms of sex would be misleading - it’s also a major regulator of sugar, fat and proteins in the human metabolism. Women also produce testosterone, in lower levels, and as well as affecting their sex drive, it helps with energy levels and general sense of well-being.

The testosterone molecule’s structure is C₁₉H₂₈O₂. It’s a member of the steroid compounds, of which the cholesterol molecule is also a member. In fact, the testosterone molecule is a shortened version of the cholesterol molecule (without the hydrocarbon tail of cholesterol).