A theory is an explanation for why or how something happens, based on careful observation, evidence, and reasoning. When scientists develop a theory, they've studied patterns, conducted experiments, and tested their ideas repeatedly. The theory of gravity explains why objects fall to the ground. The theory of evolution explains how species change over time. These theories have survived decades or centuries of testing and still hold up.

In everyday conversation, people sometimes use “theory” more loosely to mean a hunch or guess: “My theory is that someone ate the last cookie.” But in science, a theory represents our best, most reliable understanding of how something works. It's built on mountains of evidence.

A theory is different from a hypothesis, which is an educated guess you test to see if it's right. If a hypothesis passes many tests and explains a wide range of observations, it might eventually become part of a larger theory.

People sometimes say “it's just a theory” to dismiss an idea, but in science, calling something a theory actually means it's been thoroughly examined and supported by evidence.