This trope is a kind of Special Effects Failure when light objects are meant to portray other types of heavy material but fail to realistically convey the weight of that material. As the trope name suggests, this often results when fake rocks are made of styrofoam and consequently look too light to be made of stone.
The true weight of a prop is often revealed by the way it interacts with gravity. When a light prop, such as a styrofoam rock, gets dropped, it will often bounce in a way that reveals how light it actually is. A heavier object would have to be going a lot faster to bounce, sometimes fast enough that the impact would crack the material instead. The prop may also ricochet harmlessly off of actors or other scenery that, in all reality, should have been crushed by the heavy object it represents.
The way that actors handle props can also betray their true weight. Many props are intentionally created to be lighter for convenience. If an actor needs to move an object around quickly, precisely or without laborious effort, lighter props make it easier. Handling or carrying heavy objects over the course of a performance can also wear the actors out, so lighter props preserve their stamina.
It's very common for containers to be empty of their contents simply because it's not visible to the audience. Why bother refilling an actor's coffee cup between takes when he can just pretend to drink from it? But when the actor handles the cup, he can't help but betray that it's too light to be full of coffee.
Related to Hollywood Density, when the prop is too light because the storytellers never realized how heavy it actually would be.
- The boxes thrown between Bond and terrorists in Diamond's Cut are obviously empty and wouldn't hurt anyone, especially when both Bond and terrorists have guns.
- In the Parody Fic Plan 7 of 9 from Outer Space, Captain Proton denies the Space Patrol is trying to exploit Planet X because it's just a rock. "Styrofoam rock admittedly, but still a rock!"
- 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.
- Dragonball 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.
- 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.
- The end of Ghostbusters (1984) 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.
- Rocks in Mortal Kombat: Annihilation tend to bounce a lot.
- Falling rocks at the end of The Terror start floating on the water.
- 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.
- 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.
- Obvious fake rocks used in Seven Chances, where the hero performs a memorable Indy Escape down a hill.
- Played for laughs, as everything else, in the intro sequence of Pineapple Express.
- Played in-universe in Galaxy Quest as the eponymous Show Within a Show is a parody of Star Trek.
- Used in Lord of the Flies for Piggy's death scene. He's crushed by a boulder, and they try to use camera tricks to minimize the amount you can see of it being fake, but it's very obvious that it's light and he's just crouching as it hits him to make it seem like it's killing him.
- 21 Bridges: The robbers are explicitly said to be stealing 110 lbs of cocaine. Divided into four tote bags, each bag would weight 27.5 lbs. When Taylor Kitsch's character pulls his bags out of the car, however, they swing from the straps in his hands without any inertia, revealing that the props are almost weightless.
- Plan 9 from Outer Space features the infamous graveyard scene where headstones flop over easily, revealing that they're just cardboard.
- 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. Likewise in "The Perfect Mate", a Ferengi "struggles" to lift a similar container that is clearly empty.
- Star Trek: The Original Series: in the 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.
- 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.
- In El Chapulín Colorado, it's not only rocks but literally everything. El Chapulín hits an enemy with a chair, and you can actually see the styrofoam right there.
- 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.
- 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.
- Doctor Who:
- This occurred in several early 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.
- "The Dominators" features a scene where the Quarks enslave the Dulcians to do manual labour lifting rocks. There are extended scenes featuring some of the most melodramatically-acted styrofoam lifting ever committed to video. It's especially funny when the Dulcians buckle under the weight and collapse and the rocks bounce around all over the place like tennis balls.
- In "The Green Death", the Doctor has to lean against a rock wall to retrieve an egg. The rock looks fine, but there's a really loud and clear styrofoam squeak audible when he does it.
- In "Robot", the newly regenerated Fourth Doctor performs a Brick Break. The brick prop looks good, but later in the scene he bats it to the ground in a tantrum and the impact makes the distinctive sound of balsa wood.
- The Ogri in "The Stones of Blood" are a whole alien race of this. There are some amusing on-set photos◊ of Tom Baker messing about with them.
- The piece of fallen rock crushing the Master in "Logopolis". Anthony Ainley's acting does a good job of conveying the weight, but when Tom Baker moves it he clearly cannot be bothered to pretend it's heavy and just tugs it up casually with one arm.
- 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.
Tom Servo: Maybe [Guiron] can help us clean up this styrofoam.
- Viper Episode "Cold Storage". Two people are running away from an explosion. The Rocks flying around are out of Styrofoam. They have been broken and you can clearly see the white unpainted inside of the Rocks.
- Back when Live Action TV really was live, this could be a problem as you couldn't just stop and fix things. On Captain Video a struggle between the spunky sidekick and Villain of the Week tipped over one of the 'rocks', leaving its hollow construction facing the camera while the two actors 'subtly' tried to put it back while carrying on their fight.
- Blake's 7.
- "Hostage" has a hilarious scene where Travis and his two hired thugs flee in terror from an ambush involving rolling rocks that bounce across their bodies without squashing them. You'd think as they were in a BBC Quarry Blake and his team could find some genuine rocks.
- Inverted in "Time Squad", where to make the BBC Quarry look more alien they had styrofoam plants among the real rocks.
- In Takeshi's Castle, the games Avalanche and Boulder Dash involved contestants trying to avoid large styrofoam rocks that are being thrown at them. Hilarity Ensues if the contestant gets caught in a pile up.
- Parodied in Garth Marenghis Darkplace; during a scene ostensibly set in a cemetery, several of the "gravestones" are very visibly wobbling from the wind in the background. In another episode, the heroes being attacked by a telekinetic villain is replicated by hurling obvious prop items at the actors, all of which bounce off harmlessly.
- 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.
- Used in-universe in an episode of The Legend of Tarzan. Tarzan freaks out when he sees a seemingly helpless actress about to be crushed by an avalanche, and rushes to save her. However, the actors show him after the scene that the boulders were actually fake ones that are incredibly light.