In movie theaters, most movies don't come all on one film reel, they come in at least two, depending on how long the movie is. It's the projectionist's job to switch the reels.

However, you never see the movie stop just so they can put a new one on. That's because there are 2 projectors in the booth. While one is running, the projectionist loads the film onto the other.

If you look carefully during a film movie, you'll see a flicker of a circle in the top right hand corner of the screen. It lasts for 4 frames (1/6th of a second), and is a signal that a changeover is due in 12 feet of film, when it runs out. This means there is about 8 seconds until the reel's end. Then there's one more, around 1 foot. The film industry calls them "Cigarette burns" because they are basically a hole in the film, but done by a hole puncher. Also a bell on the projector goes off, which should wake up any sleeping projectionist.

At exactly the right time, one projector is turned off simultaneously as one is turned on. The movie goes on uninterrupted, and nobody notices. That's what you call a changeover.

I know someone who did this in high school, and he told me every time he saw one of those in an old movie (movie reels are taken and go to VHS, with the holes still there in the film), he'd tense up, like he missed the changeover. Plus, even though he's an adult, he still wakes up in a sweat, thinking he heard the bell and missed the changeover.

Now, most modern theaters have the whole movie on a 6-foot diameter platter or digital projection, obviating the need for the Cigarette burns, but they're still there on the film. I like 'em, maybe I have sharper eyes.

If you really want some other cool details, I'd read Fight Club, the movie extremely briefly mentions it.

Thanx to riverrun for giving me the right numbers.