Dehydration is a wonderful thing in fiction! It can make anything from creatures to base elements shrink to a easily portable size, and all you need to do to return it to the way it was before is to put a drop of water on it!
Perhaps writers used to have a lot of those expansive sponges or they're very familiar with the factoid that a person is 70% water (a house is seventy percent bricks but try throwing some bricks at some concrete, metal poles and some roofing tiles). However the origin seems to be connected to Food Pills and the idea of astronaut food. Sailors, soldiers and astronauts have needed easily transportable, highly nutritious and long lasting ways of cooking and storing food (and in the case of astronauts, something you can eat in zero gravity), when trying to think of what might be useful for life In Space!, something like this trope turns up.
In a way it's connected to the tendency in sci-fi for large, complex objects to fold or retract into small ones, such as George Jetson's car turning into a suitcase, those vanishing animal-head masks in Stargate SG-1, or the capsule items in Dragon Ball. In all of these cases, the shrunken item is easily carried, implying all that mass actually went somewhere else. At the very least, there's an overarching trope here- the idea that if you make an object smaller, it magically gets lighter too.
There is an element of Truth in Television to this trope, at least as far as foodstuffs with a high water content go, as water actually is quite dense. A 1m x 1m x 1m cube of water weighs in at a metric tonnenote so if you can remove the water from a foodstuff you can remove a significant chunk of its mass. The reconstituted product you get from adding the water back is rarely remotely as palatable as the pre-dehydration product was (though some fruits make quite enjoyable snacks as dried fruit).
- The famous later series of Smash instant potato ads are on a related theme with robotic aliens mocking backwards humans for making mashed potato the normal way when they could just use Smash which is essentially the closest humanity gets to this trope- powder to which you add water to make mash potatoes.
- In the film adaptation of Blame!, Killy is invited into a settlement that is running out of food. He hands over some rations; 1 king-sized candy bar + 20 gallons of water = 8 cubic feet of spongy "food".
- In the manga-only final story arc of Ranma ½ Akane grabs hold of a magical heat-producing staff which instantly boils all of the water out of her body, but rather than turning her into a desiccated mummy it turns her into a 6 inch tall doll. To bring her back to life (and normal size) Ranma has to soak the doll in some magical water.
- Employed in one of Carl Hurley's routines involving him going on a dehydrated food diet and losing three pounds... and then gaining five when it rained.
- The Bash Street Kids of The Beano tried making food from dehydrated mash, sausages and gravy once. It went wrong when they used far too much water and the food was bursting out of the kitchen.
- Speed Bump featured a strip with "Powdered Water: Just Add Water".
- Megamind's dehydration ray, which is impossibly harmless. If you get hit with it, you get turned into a small perfect cube (presumably equivalent to the mass of you that isn't water, although we never see what happens to the water), and then as soon as that cube gets slightly damp it transforms back into a perfect healthy human being.
- And he uses it on everything. Stray cats, bags of garbage, Minion, himself...
- The dehydrated pizza in Back to the Future Part II. We laugh when they say "Aw, grandma! Who's going to eat all that?" but then they add the water and you see this massive pizza.
- They also appear in the Back to the Future animated series, where Verne would be bullied for lunch money, admit that he only had the pills, which the bullies then knocked out of his hand and when they hit the wet ground they popped into a full turkey dinner.
- The instant food in The Fifth Element, which has to be parodying this trope - Leeloo grabs what looks like a pepper shaker, off-screen something small is heard rolling into the bowl, and after a few seconds in a microwave, it has become an entire roast chicken with vegetables.
- The 60's Batman: The Movie where all the diplomats got dehydrated. Each of them was reduced to a test tube of powder, which didn't appear to be abnormally heavy. They just added water to the powder to return them to normal (Unless you used heavy water... then the results were not so good...)
- The 60's Batman series also featured "instant costume capsules" in one episode. Drop the capsule into a cup of hot water, and it expands into a full sized Bat suit.
- Spy Kids had packets that expanded into Big Macs in a split second after being placed in a microwave-like "rehydrator".
- The Force Awakens has ration packets that contain a powdery substance. Drop the powder in a bowl, add a little water, and in seconds it becomes a big, round, muffin-thing.
- A Deleted Scene from The Black Hole has the following dialogue:
Lt. Pizer: I'm starving. What's on the menu for Christmas Eve?
V.I.N.CENT.: Dehydrated turkey, with dehydrated oyster stuffing. Also dehydrated cranberry sauce, dehydrated gravy and giblets, dehydrated sweet potatoes in dehydrated orange sauce, dehydrated vegetable salad, dehydrated mince pie, dehydrated—
Lt. Pizer: [firmly cutting him off] I envy you, V.I.N.CENT.
V.I.N.CENT.: That's not surprising. Why?
Lt. Pizer: Because you don't have any taste buds.
- In Robert A. Heinlein's short story Magic, Inc.., based in a Magitek Alternate History, someone comes up with the idea of using magic to make a tiny raincoat you can keep in a pen sided holder that grows to full size when it gets wet. The protagonist realizes that you could create a huge industry of all kinds of camping supplies or just anything that you want to carry small that you can just add water to make it full sized.
- In Eden Green and sequel New Night, humans infected with an alien needle symbiote are able to regrow entire parts of their bodies after disastrous injuries, a process that uses mostly water as raw material.
- Star Trek: The Original Series:
- An alien race from the Andromeda galaxy (the Kelvans) has the ability to "distill" people by using a device on their belts, reducing them down to a porous, softball-sized cuboctahedron. (Water not needed for restoration.)
- Subverted in another episode, when an entire starship crew (except her captain) is dehydrated and crystallized by an unknown pathogen on a planet they visited. The process is shown to be very painful and irreversibly fatal.
- In the episode of Kenan & Kel set in the year 3000, the characters buy all of their food in tablet form. When they add water to the tablets, they turn into the actual food.
- Parodied, like many things, in Batman (1966). When Bruce and Dick are trapped in a greenhouse with just a glass of water each, Bruce pulls out instant costume capsules. After dropping one in each glass and they just needed to wait a few minutes for two fully loaded costume to hydrate.
- Dungeons & Dragons. Dragon Magazine #73 had the Pill of Plentiful Water. Just add a tiny amount of water to it and it would expand and become a gallon of water.
- Chaosium's Stormbringer supplement Stormbringer Companion. When placed in water, a waterhorse wafer will swell up and become a full sized waterhorse (a demon from the Plane of Probability.
- The original Space Quest game from Sierra On-Line, featured a canister of dehydrated water (hydrogen gas). Part of the game takes place on a desert planet; you actually have to drink the water to avoid dying. You could also throw the canister at the creature Orat. His belly would swell up and he would explode.
- Subverted and mocked in The Simpsons episode " Bart Sells His Soul": Bart uses the five dollar he got for his soul to buy dinosaur shaped sponges which the page advertises as growing gigantic when made wet. Bart imagines them growing into large dinosaur sized sponges which then frighten Lisa. What he gets is a pair of sponges that get slightly bigger and then get washed by the hose down the sewer drain.
- An episode of Chip 'n Dale Rescue Rangers has Monterey Jack unwittingly swallow a piece of "dehydrated cheese" while aboard a space shuttle. It immediately inflates to full size, comically turning Monty into a large brick shape.
- In one old episode of Family Guy, the Griffin family is preparing for the Y2K Apocalypse. Peter and Lois have this conversation...
Lois: What the hell, Peter?! You just ate a year's worth of dehydrated food!
Peter: Yeah, and it was a waste of money! I'm still hungry!
(Peter drinks a glass of water and his body immediately expands to several times its size, smashing the chair he's sitting on.)
Peter: Everyone leave. I have to poop. NOW!!
- In Ren & Stimpy Ren ingests a pill labelled "Cowboy's Delight Dinner". It rehydrates inside him as a whole horse.
- Mr. Horse: No sir, I don't like it.
- The episode "House of Next Tuesday" features a dehydrated bed. Then Stimpy pees on it...
- In the Wile E. Coyote and the Road Runner cartoon "Scrambled Aches," Wile E. Coyote uses Acme Dehydrated Boulders, carrying a box to the edge of a cliff. One drop of H2O from an eye-dropper, lift boulder high above head, and Hilarity Ensues.
- An episode of Secret Squirrel had him defeat the Big Bad Wolf by trapping him in a dehydrated grandma's house (which shrunk when the sun dried it out again).
- The Looney Tunes short "Hare-way to the Stars", where Marvin deploys Instant Martians from a gumball dispenser. Bugs later takes it with him to Earth and it accidentally falls down a sewer.
Run for the hills, folks, or you'll be up to your armpits in Martians!
- In another Looney Tunes short "I Gopher You", the gopher twins see all the produce in the farm they live under sent to a food-processing plant. One of them falls into a dehydrator machine, which produces meals in tiny packets. The other uses water to rehydrate him, no worse for wear, on a platter with an apple in his mouth.
- Played straight in an episode of What's New, Scooby-Doo? where melons and other foods are dehydrated to tiny sized, and spring back to normal in a pool.
- Spongebob Squarepants: In "Tee at the Treedome", Spongebob and Patrick shrivel up to a dried-up starfish and kitchen sponge, and are restored by pouring water over them. The same thing happens again in The Spongebob Squarepants Movie when the two go to Shell City, with a fire sprinkler rehydrating them and all the other dried sea life.
- In Lilo & Stitch: The Series, all the experiment pods, golf-ball sized colored balls, are activated by water, turning into life forms of various size. In one episode, a fruit dehydrator allows for the re-dehydration of one particularly irredeemable experiment.
- Parodied when Fry chokes on a dehydrated tent by eating it in pill form and drinking some water.
- A swimming pool in "Crimes of the Hot" is filled with Dehydrated Water (Dead leaves and chlorine included). Just throw a cup of water and the pool fills up!
- An episode of the Timon & Pumbaa spinoff series of The Lion King centered around Timon getting a job at a fast food outlet, and constantly messing up an order for a disgruntled bear who makes it VERY clear that he doesn't like onions. In one such mishap, Timon realises in horror that the burger he had just served to the bear had dehydrated onions in it, and, as the bear opens his mouth to take a bite, a drop of saliva rolls off his tooth and onto the burger... Hilarity Ensues, as does more pain for Timon.
- In the Adventures of Sonic the Hedgehog episode, "Musta Been a Beautiful Baby", Tails eats some dehydrated food at a factory, then drinks a glass of water. This results in him becoming massively obese. When Sonic tries to get them away from Scratch and Grounder, Tails' massive size gets him stuck in the doorway.
- Rocko's Modern Life:
- "Hut, Sut, Raw" had Rocko, Heffer, and Filbert go on a camping trip. At one point, Rocko takes out of bag of "Instant Fire" with the instructions "Just Add Water!"
- In "Schnit-Heads," at the Sausage Works, new employee/cult member Heffer is served some kind of powdered mass for lunch. The cafeteria worker initally jokes it's dirt, and then reveals it's really powdered sausage: just add water! So he pours some water onto the powder and it instantly turns into regular sausage links. Naturally, Heffer thinks it's cool.
- In "Future Schlock", after returning from a trip to outer space, having apparently not aged in that time, Rocko and Heffer are getting used to what is, from their perspective, the future. Heffer loves it.
Heffer: I love the future.
[adds a drop of water to a pill that instantly becomes a huge sandwich]
Heffer: I love the future!