Styrofoam Rocks

Alice and Bob are on an epic adventure to find a treasure buried deep underground. They are making their way through a maze of tunnels, with only a flashlight and an ancient treasure map keeping them from getting lost. But oh no! The villain hot on their heels accidently sets off a bomb and the ground starts shaking under the heroes' feet! The ancient tunnels aren't built sturdily enough to take the tremors and the walls crumble around them!

Except the rocks that fall sort of...bounce when they hit the ground...

When shooting in a studio, you can't just bring in a truckload of rocks and drop them on the actors. Hell, even wood won't work - what if somebody is injured! The rocks could of course be done with CGI, but that would cost quite a deal, and be hard for the actors to properly portray.

What do we have left? Styrofoam!

This trope is a kind of Special Effects Failure and is applicable in situations where light objects, such as styrofoam, are meant to portray rocks, bricks or some other heavy material in a dramatic fashion, but fails to lend any weight to the seriousness of the situation.

"Tom Servo: Maybe [Guiron] can help us clean up this styrofoam."
 * In the commentary for Underworld, the movie's creators point out in a scene towards the end of the movie that some of the stone debris from someone getting thrown through a wall can be seen to float in a pool of water. Oops.
 * Dragon Ball Evolution: In the climax of the movie, the heroes are in a valley between rock pillars. Except the rocks look very light.
 * In the climax of Our Man Flint, one of Flint's fleeing girlfriends is hit by a styrofoam "boulder" and it bounces right off.
 * In the Star Trek: The Next Generation episode "Ethics", Worf's spine is broken when a cargo container falls on him. It seems almost humourous, since the way it falls and bounces indicates that it's so light it wouldn't even hurt a human, let alone a big sturdy Klingon.
 * There's another example in the TOS episode "The Return of the Archons": A melon-sized "rock" bounces off a stuntman's head and he keeps running. Apparently it wasn't supposed to hit him at all, and was left in under time pressure.
 * The opening to UHF makes fun of this, as a rock bounces right off George Newman's head mid-fantasy and does nothing to him.
 * Stargate SG-1 has an episode with a flashback which reveals how Daniel Jackson's parents were killed. Apparently, they were crushed to death under the collapsing walls of an ancient cardboard temple.
 * It was also parodied rather hilariously in the Wormhole X-Treme episode when they kept pelting the Jackson-analog with the "bricks" in multiple 'takes'.
 * Nickelodeon's Legends of the Hidden Temple and Guts were made of this trope. They were game shows for children, so it would have been disturbing if the effect were more convincing.
 * The end of Ghostbusters has debris falling from the top of a skyscraper, and in one shot you can see a rock, which had supposedly tumbled from several hundred feet in the air, fall straight down and bounce off of a wooden police barrier.
 * This occurred in the series finale of Small Wonder, when Ted gave a literal meaning to the term Cardboard Prison. He offered to pay for the damage, but the movie director said there was no money in the budget for it.
 * A common staple for Power Rangers.
 * Rocks in Mortal Kombat: Annihilation tend to bounce a lot.
 * Falling rocks at the end of The Terror start floating on the water.
 * Played with on Phineas and Ferb; When they are constructing their life-sized replica of Niagra Falls, Ferb (on the ground) picks up a boulder and throws it all the way to the top of the structure. Phineas says that even though he knew that that was papier-mâché, the throw was still very impressive.
 * In Congo rocks fell from the cave roof; some bounced and those that landed in the underground river floated.
 * The Three Stooges used this a lot, to keep up the illusion that the three had the endurance of a typical cartoon character when being hit in the head with rocks or bricks.
 * This occurred in several early Doctor Who stories, but most notably in "An Unearthly Child" and "The Daleks." It was a variation in that rocks and rock walls were made of the stuff; the DVD of "The Daleks" points out where Ian accidentally pulls off a chunk of the rock face (!). It's entirely forgivable because the show was at the time confined to Lime Grove Studios.
 * Mystery Science Theater 3000: Gamera vs. Guiron. When a building collapses on the kid heroes, the SOL crew point out how little it looks like bricks.


 * Seen occasionally in The Lord of the Rings films, as most of the sets were made of polystyrene (in fact, during production the films became the biggest consumer of polystyrene in the world). Thus, any time there are earthfalls, the rocks will fall a little too lightly.
 * A famous goof in Raiders of the Lost Ark has Indy shove a loose cubical stone the size of a compact car hood out of a wall, and the shadow shows it bouncing several times as it hits the sand.
 * During the climactic attack at the end of Disney's Swiss Family Robinson when the titular family rolls the logs down the hill at the pirates, one of the "logs" collides with the head of an unfortunate pirate and makes a quite audible and very hollow, unwooden "bonk". It's hilarious.