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A video showing a man demonstrating how to quickly block a classroom door with a chair in an active shooter situation has gone viral this week, igniting a debate about school safety in the wake of the massacre in Uvalde.

Robb Beaux tweeted the 45-second “life hack” video Sunday, in which a man in a reflective work vest gave a demonstration with a plastic-and-metal school chair.

In the recording, the man instructs his audience off camera to put a tennis ball, “some glitter, or googly eyes” on the back left leg of the chair, so that everyone knows that it is the leg needed to block the door.

The instructor then picks up the chair, wedges the back left leg into the door handle, and twists the chair backwards, leaving it suspended.

“And this thing is solid,” the man in the vest says of his makeshift barrier. “It’s not going anywhere, and that took me, what? Two seconds, if that?”

The instructor removes the chair, then demonstrates the door-blocking technique again, assuring his audience that the door could not be opened from the outside.

“It’s a pretty good trick,” he says, as someone tries — and fails — to open the door as part of the demonstration.

As of Tuesday morning, the video has been viewed more than 1.8 million times.

“It sucks I even have to share this. But just in case,” Beaux wrote in a tweet accompanying the clip.

The video was shared just days after an 18-year-old gunman stormed into Robb Elementary School in Uvalde, Texas, barricaded himself inside a classroom and proceeded to shoot dead 19 students and two teachers, while more than a dozen cops waited outside in the hallway for a janitor to unlock the door.

Some commenters have expressed skepticism that the chair trick would work, with user RXRDBOY tweeting: “So you telling me an AR15 with 100 rounds can’t shoot the knob off?”

“This is to buy time,” Robb Beaux replied to his critics. “Seconds matter in a active shooter situation. Hopefully the next time (bc there will be a next time) the police dept won’t be full of cowards like the Uvalde PD.”

Others also have stuck up for the video.

“What’s your plan?” a user dubbed Knickers demanded of a skeptic. “Given this or nothing, you choose nothing? It could work. It might. It might not. Doing nothing will NOT work. So, you get shot for sure. Not maybes. For sure. The chair classroom getting shot, maybe not.”

The slaughter in Uvalde has reignited a national debate pitting gun control against Second Amendment rights.

Republican politicians and pro-gun activists have called for armed guards and increased security measures on school campuses, with Sen. Ted Cruz (R-Texas) proposing to “harden” schools by leaving only one door open, and posting police officers at that door.

Meanwhile, Democrats, led by President Biden, have called for stricter gun control laws after a decade of failed legislative efforts to curb mass shootings in the US.

“This is Americas [sic] problem,” Twitter user James Reynolds wrote. “The UK has its issues with stuff but we absolutely do not have this problem and neither does most of the world…”

Another commenter weighed in: “We arent [sic] mad about putting extra protection in schools. Were [sic] mad that that is the ONLY thing certain Politicians are bothering to talk about. Putting armed guards and bulletproof glass in classrooms and arming teachers is not the answer.”