Testosterone is the main male sex hormone — a chemical the body makes, mostly in the testes, that causes the changes we think of as male: a deeper voice, facial and body hair, bigger muscles, and the ability to make sperm. Women's bodies make it too, in much smaller amounts, and need it. It is a scientific word, but ordinary people use it easily now, partly because of sport (athletes are tested for it, and taking extra is cheating) and partly because of health talk (older men are told their levels drop with age). There is a second, looser use you should recognise. People say testosterone to mean aggressive male energy, usually with a raised eyebrow: 'There was a lot of testosterone in that room', 'the film is pure testosterone'. That is a comment on behaviour — loud, competitive, showing off — not a claim about anyone's blood. Journalists especially like this use. It is uncountable: say 'testosterone', 'high testosterone', 'a testosterone level', but never 'a testosterone'. Stress the third syllable: te-STOS-te-rone.