Why do some medicines work and others don't? How do we know the Earth is billions of years old, or that germs cause disease? The answer in each case is not that an authority said so, but that people asked questions in a particular way and checked the answers against the world. That way of asking and checking is what we call the scientific method. This course explains how it works, where it came from, how it goes wrong and how it corrects itself, and why it matters to every citizen, not only to people in lab coats.

A way of not fooling yourself

In a 1974 graduation speech, the physicist Richard Feynman put the heart of science in a single rule: do not deceive yourself, because you are the person most easily deceived. Human beings are very good at seeing patterns, remembering hits and forgetting misses, and believing what they would like to be true. The scientific method is a set of habits and tools designed to catch those errors before they become beliefs. It does not make scientists smarter than other people. It makes their mistakes easier to find.

A cycle, not a recipe

School textbooks often present the method as five or six fixed steps. Real science is messier, but the steps capture a genuine cycle:

• Observe something that needs explaining.
• Ask a question about it.
• Propose a hypothesis, a possible explanation.
• Work out a prediction: if the hypothesis is true, what should we see?
• Test the prediction by experiment or careful observation.
• Analyze the results and share them so others can check.

The results usually raise new questions, so the cycle starts again. Scientists jump between steps, work backward from surprising data, and sometimes stumble on discoveries by accident. What stays constant is the commitment to let evidence decide.

What makes a question scientific

A question is scientific when some possible observation could help answer it. “Does this fertilizer make tomatoes grow larger?” is scientific: we can grow tomatoes with and without it and measure them. “Are tomatoes more beautiful than roses?” is not, because no measurement could settle it. Neither is “Should we spend public money on tomato research?”, which depends on values as well as facts. Science can inform such questions by telling us what is likely to happen, but it cannot by itself tell us what we ought to want. That limit is not a weakness; it is a sign that science knows its job.

Science also looks for natural explanations, ones that can be tested. Saying that an invisible spirit makes the tomatoes grow, in a way that no test could ever detect, may or may not be true, but it lies outside what the method can examine.

Fact, law, theory

Three words cause endless confusion. A fact is a repeatable observation, such as “dropped objects fall.” A law is a description of a pattern, often a mathematical one, such as Newton's law of gravitation, which tells us how strongly two masses attract. A theory, in science, is a well-tested explanation that ties many facts and laws together, such as the germ theory of disease or the theory of plate tectonics. In everyday speech “theory” means a guess. In science it is close to the opposite: a theory is the strongest kind of knowledge science offers. Theories do not graduate into laws; they do different jobs.

Science as a community

No single scientist is reliably objective. In 1942 the sociologist Robert Merton described the norms that make the community more reliable than its members. Findings belong to everyone (communalism). Claims are judged by evidence, not by the claimant's nationality, race or fame (universalism). Scientists should seek truth rather than personal gain (disinterestedness). And every claim, however respected, stays open to challenge (organized skepticism). Real scientists fall short of these ideals, as later levels show. But when the community as a whole keeps them, individual errors tend to be caught.

Worked example: the phone that dies in the cold

Suppose you notice that your phone battery drops sharply on winter mornings when you walk to work. Run the cycle.

• Observation: the charge falls fast on cold days.
• Question: is the cold itself the cause, or something else, such as using maps more on those walks?
• Hypothesis: low temperature reduces the battery's usable charge.
• Prediction: if so, the phone should lose charge faster in the cold even when it is doing the same task as on a warm day.
• Test: run the same video for thirty minutes, once indoors and once outside on a cold day, starting from the same charge. Repeat a few times.
• Analysis: compare the drops. If the cold runs lose much more, the hypothesis survives. If not, look at the other explanation.

Notice what made this scientific. You did not simply trust your impression; you designed a comparison that could have proved you wrong. That single habit, applied with more care and more people, is what the rest of this course builds on. (For the record, chemists confirm the hypothesis: cold slows the chemical reactions inside lithium-ion batteries.)