Hypothesis, Theory, or Law?

By the end of this lesson, you will be able to tell the difference between a scientific hypothesis, a scientific theory, and a scientific law, and explain why each one matters.

Imagine you walk out to your car on a hot Texas afternoon in August and notice that your bicycle tire looks puffier than it did that morning. You wonder why. Your first idea might be, "If the air inside the tire gets hotter, then the tire will swell because hot air pushes harder." That idea is a hypothesis, which is a testable explanation for something you observe that you can check with an investigation. A hypothesis is not a wild guess, because it is based on what you already know, but it is also not proven. You could test it by measuring the tire pressure in the cool morning and again in the hot afternoon.

Now suppose thousands of scientists have measured gases for over two hundred years and found the same pattern every single time. When a relationship in nature happens so consistently that it can be described with a pattern or an equation, scientists call it a law, which describes what happens but does not explain why.

A theory is different. A theory is a well tested explanation of why something happens, supported by huge amounts of evidence from many investigations. For the warm tire, the explanation is that gases are made of tiny moving particles that speed up and hit the tire walls harder when heated. That explanation is part of a theory.

So a hypothesis is a testable idea, a law describes a pattern, and a theory explains the cause. Notice that a hypothesis never "grows up" into a law, and a theory is not a shaky guess. In science, a theory is one of the strongest kinds of knowledge we have.

Try it: if you state that darker cars get hotter in the school parking lot than lighter cars, is that a hypothesis, a theory, or a law?