A factoid is something that sounds like a fact but isn't actually true. The word was popularized by writer Norman Mailer in 1973 to describe false information that gets repeated so often that people start believing it. For example, someone might tell you that people swallow eight spiders per year in their sleep (a common factoid that scientists say isn't true), or that goldfish have three-second memories (another factoid: they actually remember things for months).

Confusingly, the word has also come to mean a small, interesting piece of true information, like “Did you know octopuses have three hearts?” Many people now use factoid this way, even though it contradicts the original meaning. When you hear someone share a factoid, it's worth checking whether it's actually accurate, especially if it sounds surprising or gets repeated a lot on social media.

This confusion makes factoid a perfect example of something that seems like a fact but might not be reliable at all.