An anachronism is something that appears in the wrong time period. When a movie about ancient Rome shows a gladiator wearing a wristwatch, that's an anachronism: wristwatches didn't exist 2,000 years ago. When someone writes a story about medieval knights and has them eating corn on the cob, that's another anachronism, since corn wasn't grown in Europe until after explorers brought it from the Americas.

Writers, filmmakers, and historians try to avoid anachronisms because they break the illusion of being in a different era. If you're reading about George Washington and suddenly he pulls out a cell phone, you'd immediately know something was wrong.

Sometimes anachronisms happen by accident, like when a movie about the 1800s accidentally shows a modern car in the background. Other times, creators include them on purpose for humor or to make a point. The key is that an anachronism is something out of sync with its time period: it belongs somewhere else on history's timeline. Finding anachronisms in movies or books can be like a treasure hunt for details that don't quite fit.