evolution

Evolution is the way living things slowly change over many generations. The traits that get passed down through families — things like color, size, or how a body works — shift little by little as time goes on, so that descendants can end up quite different from their distant ancestors.

The key thing to grasp: evolution happens to whole groups, not to single creatures. One rabbit doesn't evolve during its life; rather, over thousands of generations the rabbit population as a whole comes to look and behave differently. Natural selection — where traits that help survival and reproduction become more common — is one driver, but not the only one. Chance also plays a role (some genes spread or vanish by luck), and brand-new variations arise through mutation.

Evolution also tells us that all living things — bacteria, oak trees, whales, you — are distant relatives, branching from shared ancestors over billions of years. And when scientists call it a "theory," they don't mean a hunch. In science, a theory is a well-tested explanation backed by mountains of evidence, much like the theory of gravity.