A fact is something you can check, and an opinion is what a person thinks. Telling them apart is the first step in finding out what is true. Here is a surprise, though. Whether something is a fact or an opinion does not depend on whether it is right, because a false claim that could be checked is still a claim of fact.

Take the horned helmet. You may have heard that Vikings wore helmets with horns when they fought, and that is a claim of fact, so it can be checked. When people checked, they found that Vikings did not wear horned helmets, and we have no proof that they did. The made-up picture comes from a costume for an opera in 1876.

Opinions work in a different way. Experts who study the same thing can still have different opinions about it. An opinion can be backed up with good reasons, but a reason is not the same as a fact you can check, and that is why people can disagree and still both be thinking carefully.

Photos can fool people, too. In 1917 two cousins in a village in England took photos that seemed to show tiny fairies. Many people looked at them and believed they were real, even experts at a camera company, and the photos fooled many people for years. Then, in 1983, the two women who made them said the fairies were fakes.

One of the women said she had drawn the fairies and that they were stuck on twigs with hat pins. The fairies were paper cut-outs held up with hat pins, and the whole thing had been a hoax. So how can you check a claim? Start with a question: who says it, and how do they know?

A good source should say where its facts came from. It also helps to ask why something was made, because a page might be trying to teach you or it might be trying to sell you something. Then check a second place, since a true fact can be found in more than one good place. If a claim has no clear source, try to find the same fact somewhere else before you trust it.

Look for the date, too. A page may say when it was written or last updated, and old facts can go out of date. Even newspapers can get it wrong. In 1835 a New York newspaper printed that an astronomer had seen life on the Moon, but the story was made up. It was printed in a cheap newspaper and then reprinted by papers around the country.

People can also fool themselves. Confirmation bias is when people look for facts that fit what they already think and ignore the rest. In 1912 a skull was found that seemed to match what scientists hoped to find, so many believed it. It fooled scientists for about 40 years, until tests in 1953 showed that the jaw was an ape's.

Many myths last because they sound right. You cannot see the Great Wall of China from space with just your eyes. People use almost every part of their brain, not just a tenth of it, and people knew the Earth was round long before Columbus sailed. So when you hear a big claim, ask who says it, and check a second place.

Words to know

• opinion: a personal view
• hoax: a trick
• source: fact giver
• reprinted: printed again