Warfare became infinitely more gruesome during World War II thanks to some rather incredible advances in technology. But a very deadly trick deployed by the builders of a bunker in Normandy used nothing more than the art of misdirection — quite literally.

A hole, disguised as an air vent, would seem the perfect place for an advancing enemy to pop a grenade. However, the tosser would come to regret that when the explosive simply dropped through a small pipe and fell out at the assailant’s feet.

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A post shared by Normandy Bunkers (@normandybunkers)

Posted by Instagrammer @NormandyBunkers, a video demonstrates exactly how the anti-grenade trick would have worked.

“Instead of falling inside the bunker, the construction meant the explosives would fall at the feet of the attackers,” the caption reads. “Anyone assaulting this position and dropping a grenade into the vents would be in for a big surprise.”

The contraption brings to mind that episode of Tom & Jerry, in which Tom the cat puts the end of a rifle into a hole in the wall, which bends the butt of the gun right back out toward his face.

The account runners have been sharing posts about the history and architecture of several Normandy landmarks associated with World War II.

“We have a friend who owns a complex of bunkers behind Utah Beach and became fascinated with the history surrounding the site, it’s architecture for a purpose, and the archaeological finds that came with its excavation,” they told Military Times.

“We originally planned to chronicle the work but got hooked on the whole ‘Atlantikwall’ structures. We now have over 40,000 pics, videos, and drone footage from D-Day landing beaches, Normandy bunker sites and beyond, all the way from Le Mont Saint Michel to Dunkirk.”

Originally published by Military Times, our sister publication.