Watch this 1999 video by psychologists Christopher Chabris and Daniel Simons. You won’t be sorry, I promise.

Did you get the number of ball passes correct? I really focused, and I did count correctly.

For those who didn’t see the 300-pound gorilla in the room, which I did not, this phenomenon is entirely normal. Did you ever not notice a friend’s new haircut? Or not realize that a sibling had gained weight over the last 10 years? If so, it’s not because you’re too self-involved. According to researchers, it’s how our brains are wired. It’s called “change blindness,” and it’s the phenomenon of not noticing something—even a 300-pound gorilla—when it stares us in the face.

There’s also “inattentional blindness,” when we don’t see the difference because we’re not looking for it. In the video, if didn’t see the gorilla, it’s because you were told to count the number of ball passes. If you’d been told to look for the gorilla, you’d have seen it. Which I did, easily, when I played the video back.

These blindnesses are shortcuts our brains take. There’s too much information to process everything constantly, so the brain fills in the space we don’t much care about. The brain assumes. And we know how assumptions can sometimes get us into trouble!

While inattentional blindness can be fun when it’s about gorilla videos or video games, it can have serious consequences, because if you’re not looking for gorillas, you won’t see them. So if you’re not consciously looking for motorcycles or dogs in the road, you might miss those, too.

So be careful out there! You never know when a 300-pound gorilla will be in the room.