While the machete-wielding monster or ghost of a murdered mother are fiction, our brains react as if we were in the movies with those monsters.

What happens, according to Reactions — the American Chemical Society’s YouTube series — is that the brain perceives fear, regardless of how real it actually is. We can tell ourselves it’s just a movie, but our brains won’t listen.

When scared, the thalamus sends glutamate to the amygdala, which then transmits the glutamate to the periaqueductal grey. That’s when we get scared.

The signal then goes to the hypothalamus, which triggers a “fight or flight” reaction. That reaction pumps adrenaline, glucose, and cortisol throughout the body, to provide energy in the high-stakes situation.

Even in the relative safety of a theater, elbow-deep in popcorn, our brain prepares us for the worst-case scenario.

Reactions also explains why we scream and why the characters’ on-screen screaming triggers us, as well. Screams are actually “a primordial automatic function of the body” that the brain registers as distinct from language. While language is comprehended in the temporal lobe, screams go the same amygdala that jumpstarts the fear process. Whether it’s preparing for our own troubles or helping others, screaming is the first indicator of danger.

The video also describes the pain and death processes, which are not so replicable from watching a movie. Pain occurs when nerves send signals to the thalamus, telling the body “yep, this hurts a whole lot.” We’re not likely to experience physical discomfort the same way just from watching the movie.

Finally, after all the pain and fear is gone, there’s death. First, there’s clinical death: your heart has stopped beating and you’ve stopped breathing. The brain, though, potentially enters “a hyperstate of perceptual neural activity.” Although it’s a brief moment in real time, some believe that the perceived length could actually be something like the afterlife.

That could explain why people report seeing visions of Heaven after near-death experiences. After that clinical death, of course, comes biological death, when the brain shuts down. As far as we know, not even a zombie can come back from that.

So when you’re watching a slasher flick in the days before Halloween, don’t get embarrassed when your pants get wet and your hair stands on edge. It’s a natural reaction. In fact, you should get scared.