Biology and its branches: what counts as life?

A candle flame grows, moves and needs oxygen to keep going. That sounds alive. Yet no biology book ever calls fire a living thing. So what actually decides the label?

Biology is the study of living things, and it also studies the non-living things, like water and soil, that affect life. Biologists mostly ask how life works, leaving “why is there life” to philosophers.

So biologists use nine practical characteristics instead of one neat definition. An object earns the label “living” only when it has every single one at once, not just a few impressive ones.

Biology is also too big for one person to study fully, so it splits into branches. You already know Ecology, Physiology and Genetics; this lesson adds four more you'll meet all year.