Some false ideas are so common that most people have heard them. You may have heard that Vikings wore helmets with horns, but Vikings did not wear horned helmets when they fought, and we have no proof that they did. The picture comes from a costume for an opera in 1876. You may have heard that people use only a tenth of their brain. Brain scans show that when you talk, walk or listen to music, you use almost every region of your brain.

Another myth says you can see the Great Wall of China from space. With just your eyes, you cannot. Photographing the wall from space took a very strong zoom lens. Next come stories of fakes that fooled many people.

In 1917 two cousins in a village in England took photos that seemed to show tiny fairies. Many people believed they were real, even experts at a camera company. The photos fooled many people for years. In 1983 the two women who made them said the fairies were fakes. One said she had drawn them and stuck them on twigs with hat pins.

A skull and jaw found in England in 1912 were said to be an early human relative, and the find fooled scientists for about 40 years. It was a fake: a human skull put with an orangutan jaw. Some of the people who looked at Piltdown Man were respected scientists, and they were fooled too. The skull seemed to match what scientists hoped to find, so many believed it. In 1953 scientists tested the bones, and the tests showed that the jaw was an ape's.

In 1835 a New York newspaper printed that an astronomer had seen life on the Moon. The story was made up. It said that winged man-bats and other creatures lived there, and that a giant new telescope showed them. The story was printed in a cheap newspaper and was reprinted by papers around the country, so a made-up story travelled far.

Newspaper stories can exaggerate. After a 1938 radio play about Martians, newspapers wrote that many people panicked. Later writers found the panic was much smaller. In 1938 about 5,000 homes were asked what radio show they were listening to, and only 2 percent said the Martian play or one like it.

Then there is the conspiracy theory. A conspiracy theory is an idea that a secret plan by powerful people explains an event. It usually has no solid evidence that anyone can check. A claim with nothing you can check behind it deserves a lot of care.

Good checking is a habit. Look at who wrote a claim, why, and when. A first-hand record, like a diary or a letter from the time, is called a primary source. An encyclopedia is usually written by scholars, and it collects what is known from primary and secondary sources. If a claim has no clear source, try to find the same fact somewhere else before you trust it.

Confirmation bias is when people look for facts that fit what they already think and ignore the rest. The Piltdown skull seemed to match what scientists hoped to find, and so many believed it. So when a story fits what you hope is true, ask who says it and how they know, and check a second place.

Words to know

• fake: not real
• myth: a false idea
• exaggerate: make bigger
• conspiracy: secret plan
• primary: first-hand