What Do You Mean It's Phlebotinum?

It's not easy being an alien on Earth. As if the usual shenanigans weren't enough, now your ship is critically low on Phlebotinum, and all these benighted savages use is oil, gas and electricity... But hey, what's this? Your sensor-bot beeps loudly! The natives do have phlebotinum - they just don't know it. Excitedly you tell your native friends about your discovery, but they just look strangely at you. "What Do You Mean, It's Phlebotinum?", they ask. "That's PIZZA!"

Truly, these Earthlings are a silly lot.

Compare Mundane Solution, Worthless Yellow Rocks, Solid Gold Poop, MacGyvering, Powered by a Forsaken Child.

To truly fit in this trope, the material should be of immediate use -- serve as fuel, weapons, medicine, field repair parts, etc.

If it's valuable as raw material or for monetary value, it's very close to an inverted Worthless Yellow Rocks, and should first be considered for addition there.

Comic Books

 * Some incarnations of Venom of Spider-Man fame feed off of a chemical found in the human brain. Eventually Eddie and he discover that this same chemical just prominent enough in chocolate bars to sustain him.
 * The 1989 Teenage Mutant Ninja Turtles once helped a stranded alien to fix his spaceship and get home. At the end of the episode, he was only missing the ship's fuel source, "grutnyp". It turned out to be pizza.
 * Similar to the Back to The Future example, "makers" (nanomachine hubs that can assemble almost anything) in Transmetropolitan can run either on "base blocks" of transmutable matter... or on garbage. People are shown collecting street garbage before the garbagemen can come clean the streets, so that they can run their makers cheaply.
 * Makers can work on anything. It's just that garbage is free. The advantage of "base blocks" is that they're hyperdense, which means a lot of matter to be used, so a maker can run on one for a long time.

Film
"Frank: Wait, that's gold."
 * In Back to The Future, the "Mr. Fusion" attachment causes everything to become this — you can run your car on used coffee grounds and banana peels, and throw away that useless plutonium.
 * Only when it's in flight mode, and only when it's traveling through time. It needs gasoline for a trip to the corner drugstore. Presumably the Mr. Fusion isn't compatible with the engine.
 * Which is a bit of Fridge Logic, because apparently, in the future, people have 'hover conversions' of their older cars to make the car fly through the air solely by electricity that Mr. Fusion generate for free...but for some reason leave their internal combustion engines intact to drive the cars down the streets. Perhaps gasoline is also free, but wouldn't it be rather difficult to make the wheels tilt like that and keep the gasoline-powered engine and transmission hooked up, instead of just stripping out all that and attaching electric motors to each wheel? (And removing the engine would create a better place to put Mr. Fusion than sticking out of the back of the car.)
 * Alternately, Mr. Fusion just powered the time circuits, and the hover ability somehow worked off the gasoline engine, especially when we see the smooth transitions between driving and flying. Although this seems to be even an odder premise.
 * In The Cat from Outer Space, the title character is looking for a few kilograms of a common (from his point of view) but unknown (to humans) substance to fix his spacecraft. Cue The Reveal:


 * In Monsters, Inc.., the monster civilization gets all its energy from... the screams of children.
 * In My Favorite Martian, the eponymous Martian's spaceship has broken down, and is now missing a critical engine part. Much agonizing ensues as the Martian despairs of creating a new one... until he bothers explaining what it does. It's an alternator. He's then able to repair his ship simply by ripping an alternator out of a car and plugging it in.
 * In the Carl Reiner movie Spirit of '76, a time machine sets off for 1776, but runs out of the fuel tetrahydrozeline, and ends up in bicentennial year 1976 instead; fortunately, they learn that tetrahydrozeline is the active ingredient in Visine eyedrops.

Live-Action TV
"Alien Woman: It's only onlux!"
 * An episode of Captain Kangaroo featured a stranded alien who desperately needed a "glump-full of gleedle" to fuel his ship. It turned out to be a glassful of milk.
 * In Farscape, Crichton searches for a complex chemical compound as medicine for Moya. It turns out to be the local equivalent of salt on the planet on which they've landed.


 * In Married... with Children, Al's used socks keep getting stolen by space aliens. By the end of the episode, he discovers that they're needed for spaceship fuel.
 * Apparently, his foot odor was Just. That. Powerful.
 * Inverted in the Star Trek episode "City on the Edge of Forever", where Spock and Kirk are marooned on 1932 Earth, but Spock says that he "only" needs a fair-sized block of platinum in order to build a computer to prevent Godwin's Law of Time Travel from taking effect. Meanwhile, their job only pays 15 cents an hour. Spock doesn't know that platinum is fairly rare on Earth, and is completely unobtainable in such quantities.
 * The Tales from the Darkside episode "Barter" (a parody of I Love Lucy) features an alien who needs ammonia desperately (he drinks it, so it's either food or medicine to him) and is willing to trade for it.

Magazines

 * One issue of Mad (back when it was still an anthology) had an alien whose planet was destroyed by unstoppable goo. Turns out it's strawberry Jell-O. He freaks out on realizing it.

Newspaper Comics

 * In Piranha Club, main character Ernie works at a horrible fast-food joint. A foreign (Japanese, IIRC) competitor realises that their fries are never as tasty, and resorts to espionage to find out the secret. It turns out that there is grease from Ernie's incredibly old DeSoto in the frier. Cue the spies trying to buy, borrow or steal Ernie's car - which he loves and won't part with.

Puppet Shows

 * In an episode of Lamb Chop's Play Along, the alien Zarc needs "zappelmeis" to fuel his rocketship and get back home to Yzarc. As it turns out, "zappelmeis" is apple juice.

Western Animation

 * Subverted in one episode of Futurama. Fry finds himself with a large heap of money and uses much of it to buy the last can of anchovies in existence, bidding against the megamogul Mom. Mom, seing the amount he pays for it, reasons that the anchovies must have some special property, like their oil being able to lubricate every robot in the world forever, but as it turns out, Fry just likes anchovies a lot.
 * In one episode of Mighty Max, it turns out that aliens want Earth's toxic and radioactive waste; it's apparently an extremely valuable commodity on their home planet. Win-win for Earth, Max and the aliens.
 * The premise of Sheep in The Big City consists of the military pursuing the eponymous sheep in order to power their “sheep-powered raygun“.
 * Spider-Man and His Amazing Friends had an episode where travelers from the future needed three substances to repair their ship. Spiderman's friends identified two of them as rare compounds found in lava and deep ice water, respectively... the third turned out to be ivory.

Unsorted

 * There's a short story about a friendly alien coming to earth and befriending some human children. He is seeking a legendary device that somehow knows how to keep hot things hot and cold things cold. He leaves happily with a thermos.
 * Is the alien a Letonian, by any chance?
 * How can one travel through space without this knowledge?