When someone says something, how can you tell if it is true? A fact is something you can check. An opinion is what a person thinks. Ask who says it and how they know. Look at who wrote a claim, why, and when. Ask what the writer's point of view is, and whose point of view is shown.

Next, look at the source. A first-hand record, like a diary or a letter from the time, is called a primary source. Secondary sources talk about primary sources. They comment on, sum up or explain what the first-hand records say. In science, a primary source can be a report in which scientists describe their own research.

What if a claim has no clear source? Try to find the same fact somewhere else before you trust it. A true fact can be found in more than one good place. It is also good to ask why something was made. Is it trying to teach you, or to sell you something? That is the way to check in a second place.

A story from 1835 shows why checking matters. 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. It also said a giant new telescope showed them.

Even experts can be fooled. A skull found in 1912 seemed to match what scientists hoped to find, so many believed it. Some were respected scientists, and they were fooled too. It fooled people for about 40 years. Then tests can show a claim is wrong. In 1953 tests showed that the Piltdown jaw was an ape's.

Newspaper stories can exaggerate, which means they can make a thing sound bigger than it was. In 1938 there was a radio play about Martians. After it, newspapers wrote that many people panicked. Later writers found that the panic was much smaller than the papers said.

Some old ideas are false. People knew the Earth was round long before Columbus sailed. Columbus used a guess for the size of the Earth that was too small by about a third. A much better estimate already existed. You may have heard that Columbus proved the Earth is round. In fact, people knew long before. When you hear a big claim, ask who says it and how they know.

Science can change an answer when there is new evidence. Long ago Pluto was called the ninth planet. Pluto was moved out of the planet group in 2006. Today scientists call it a dwarf planet. Experts who study the same thing can still have different opinions, so ask who says it, ask how they know, and check a second place.

Words to know

• claim: a thing said
• primary: first-hand
• secondary: second-hand
• evidence: facts that show
• exaggerate: make bigger

The short version

To check if something is true, ask who says it and how they know, then look in a second place. A true fact can be found in more than one good place. Tests and new evidence can show a claim is wrong or change an answer.

Quick facts

• Ask who says it and how they know.
• A true fact can be found in more than one good place.
• A primary source is a first-hand record, like a diary or a letter from the time.
• In 1953 tests showed that the Piltdown jaw was an ape's.
• Pluto was moved out of the planet group in 2006.