Victor: What's that, brake fluid? Diethylene glycol isn't gonna burn fast enough to do anything! Michael:(taping bottles together and tossing them out a car window) It will, if it's mixed with chlorine dioxide, and some other stuff.
As long as there are books, movies, television, and video games, people will always try to recreate everything in their favorite work.
Unfortunately, many works contain explosives, including homemade varieties. Even if the viewer could be trusted to recreate those without blowing themselves up, society is highly against people using homemade explosives. (We've had some bad experiences.) High-profile media, after a certain date, must respect this or gain the wrath of Moral Guardians or worse.
So, to avoid liability issues and criminal charges, some critical ingredient for the explosive is removed, replaced (with something less volatile) or referred to vaguely (as "stuff", "my secret ingredient", or similar) to prevent disasters like Your Head A Splode or Apocalypse How.
Occasionally happens with other types of weapons of mass destruction.
Speculative Fiction will usually make the "other stuff" Unobtanium or some real life substance that the average person is highly unlikely to ever come across (e.g. Uranium, Antimatter). See also Don't Try This at Home.
Compare Secret Ingredient, which is about being tasty rather than explosive. Contrast Noodle Implements, where you know exactly what's going to be used, and absolutely nothing about how.
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Anime and Manga
An episode of Pani Poni Dash! include a bomb, which two of the schoolgirls set to disarming. They list up the components as they do, except they're all censored by a Sound Effect Bleep. The translation notes snarkily mention that while they could list the components, "no way are we going to teach a bunch of Otaku to make bombs. You'll have to look this stuff up on the internet like everybody else."
Parodied in strip during the Last LaughCrisis Crossover in The DCU. The strip had The Joker explaining how to make his lethal Joker venom but censored out the names of various ingredients but left in comments like "You'll need to go to the hardware store for that". The joke, of course, being that you couldn't make the entirely fictitious Joker venom even if you did know what it contained.
In an issue of G.I. Joe: Special Missions, Lightfoot explains how how he is MacGyvering a fuel-air explosive out of supplies found in an enemy bunker. However, the panels have censor boxes placed over them so the reader cannot see what he is actually doing.
"A few household chemicals in the proper proportions."
The drugs that Zeke makes in The Faculty are made from "caffeine pills and some other household shit." The Other Stuff in this case is likely NaCl and dehydrated Placebo according to how the drugs affect the aliens, unless it's a case of Bizarre Alien Biology.
Hollow Man has the Invisible Man creating nitroglycerin in the matter of five minutes or so (when it's such a volatile liquid you have to measure both temperature and quantities so the nitro doesn't explode while you're mixing it). And the movie doesn't show nitric acid among the substances he gets for the nitro (the bold there was an indicative to how crucial it is).
In the Terminator's 'verse, apparently you can make plastic explosive from moth balls, corn syrup and ammonia.
In Superman III, a supercomputer determines the exact ingredients of kryptonite — except that one of the ingredients is categorized as "unknown." So Gus Gorman substitutes cigarette tar. This substitution results in what is, effectively, red kryptonite!
In Excalibur Guinevere prepares a "healing" cake for Arthur. When he asks what's in in, she coyly replies, "Fresh, unborn grain, flavored with rose petals. The rest is secret."
In Peter Pan, the explanation that the children need to be sprinkled with "fairy dust" in order to fly was added by Barrie so that children would not jump out of windows thinking that they could fly if they believe they could.
If you believe the Conspiracy Theory that says it's a plot to get anarchists to kill themselves, it's actually an inversion.
One of Chris Ryan's books contains an aversion - he tells the reader exactly how to make and arm deadly petrol bombs, using nothing but soapsuds, petrol, orange juice and fuse.
In the afterword to Tom Clancy's The Sum of All Fears (book, not movie), Clancy admits to fudging some details of the workings and construction of nuclear weapons, in an effort to not help anyone with unkind intentions involving nukes (though he also acknowledges, if somewhat cynically, it probably won't actually stop anything).
The author of The Salvation War intentionally fudged the workings of nuclear weapons, and when one of his readers pointed out the error, he said it was standard procedure.
It's not explosive, but it is dangerous: "moon tea" in A Song of Ice and Fire is based on natural abortifacients such as tansy and pennyroyal, which were historically used, but produced nasty side effects at best and would straight-up kill a woman if the mixture was even slightly off. George R R Martin "added a few fantasy touches" because he didn't want anybody trying this at home. In-universe, using moon tea is a last resort, and Hoster Tully's use of it to end Lysa's premarital pregnancy is subtly implied to be responsible for her lifelong fertility problems.
The Inn At the Crossroads ASOIAF cooking site made a recipe for tansy tea, noting that: "Nobody should drink this tea for its Westerosi purpose. In fact, nobody should drink this tea at all..."
Similarly, Diana Gabaldon's Outlander series is factually based but vague about abortifacents and penicillin, lest those playing along at home want to try, while perfectly candid about other medical subjects.
Some have argued that the infamous The Poor Mans James Bond by Kurt Saxon falls under this category, as it consists of badly photocopied and inaccurate pages about how to make your own explosives and other home-made devices of mayhem.
Michael Crichton's book A Case of Need had a note indicating that the relatively simple synthesis of LSD from lysergic acid had been removed due to legal concerns.
Discworld: "It's made of apples. Well, mainly apples." The substance in question? Scumble.
Poked fun at in L. Frank Baum's The Magic of Oz, in which one of the characters figures out how to pronounce the mysterious magical word, "pyrzqxgl". The narrator observes that he wouldn't dare write down the proper way to pronounce this word, lest it fall into the wrong hands, and advices the reader not to try to pronounce "pyrzqxgl" the right way, for fear of accidents.
The Stephen King short story Dolan's Cadillac includes instructions on how to hot wire a car, but as King mentions in his notes, he left out some steps.
The hacking in The Casual Vacancy is simply described as "SQL injection", which is a real hacking method, but the details of how one actually does SQL injection are not explained. (However, anyone who reads xkcd already knows how to do that.)
In Armadale by Wilkie Collins, the chemicals used to create a poisonous gas (probably carbon monoxide) are described only as 'Our Stout Friend' and 'a certain mineral Substance'.
Interestingly, the show averts or ignores this trope for the most part. In fact, the commentary for the episode "Lesser Evil", containing the trope naming line, says the line was only added because the explosion was too big for the stated ingredient to produce, and not to prevent others from duplicating the explosive.
For a straight example, there was the time in "Family Business" when Fiona made homemade explosives that looked and acted like C-4 using "spackle, petroleum jelly, and a bunch of other things I don't even wanna know about."
The Blue Heelers episode "Kicking Over the Traces" refers to an online guide to, essentially, being a terrorist, from guides to bombs and how to make weapons to how to be the giggest anarchist you possibly can. PJ doesn't call it by its real name, instead he calls it the Anarchist's Handbook. Several times (such as when Tahni and Ryan torch Tom's car) it's described how it was done...with omissions.
MythBusters omits bits of information concerning the exact chemical explosives they're using for various experiments.
There was an episode involving thermite, testing a theory of what might have happened to the Hindenburg. They do mention the two major reactants (aluminum powder and iron oxide) but censor some additional materials needed to make them react when exposed to flame. Lampshaded when Adam held up a pair of chemical bottles with the names blurred out.
Adam: This ingredient is made of blur. Ha! And this has blur in it too. Blur is very dangerous; you don't want to mix blur with blur.
Another episode censored ingredients with animal noises, leading to a remark from the narrator about the reaction you get when you "add donkey to rooster."
They also did this for a non-explosive myth, where they managed to create facsimiles of fingerprints which would pass biometric scanners based on a copy of an actual print lifted off a 2D surface.
When they tested Kirk's improvised cannon, an important step was left out when they made their black powder. Of course, that same step was left out of the original Star Trek episode, so one could argue they were just being authentic.
They also omit critical steps when demonstrating activities such as lockpicking and other theft-oriented skills.
In an "Evening with Adam and Jamie" in NC, they revealed that one of two episodes they outright refused to do involved a myth of an explosive mix of such common, easy-to-access materials with such destructive results, that they locked up all of the footage and swore never to go into much detail again. Even getting the above little info took many people's continued nagging.
The creators of Breaking Bad had a real live DEA agent come in to teach the writers how to actually cook meth, so that they could then carefully (and invisibly) subvert the processes shown on screen, retaining verisimilitude while actually offering directions on how to produce an over-the-counter nasal decongestant. Also, the difficulty of procuring the raw materials (which include some of the most highly DEA-controlled chemicals there are) means that anyone who tries to copy the TV show (presumably because they have a stuffy nose) will be facing down the police in fairly short order.
Leslie Fish's Black Powder and Alcohol is a survivalist song about how to make, well, what it says. The details are probably too vague to be helpful, but she is honestly trying.
In Driver: San Francisco, ammonia is correctly identified as an ingredient in the production of hydrogen cyanide, using platinum as a catalyst. However, the proper procedure is never identified beyond "if you knew what you were doing". They needn't have bothered, as the method for creating hydrogen cyanide is beyond the capabilities of most people.
The Last of Us has survivalist training manuals (or rather, scattered torn-out pages of them), which, when collected, provide Joel with handy-dandy information on how to survive in a post-apocalyptic wasteland. The pages about treating injuries more effectively have some actually pretty useful information on splints, tourniquets and the like. The pages about making smoke bombs, tying knots on weapons, sharpening shivs, and improving the construction of molotov cocktails all noticeably trail off with "..."s.
The Doctor from Scream of the Shalka makes a bomb out of a trash-can filled with fertilizer, newspaper and..."a little something" white and powdery from a bag in his pocket. The most likely candidate is probably aluminum powder, but it's hard to say.
ThisCivil Protection video parodies, then subverts, this trope. One of the characters is very careful not to say anything about how to build bombs, and the other says Don't Try This at Home whilst explaining exactly what not to do. When asked why he's explaining how to make napalm, he says that he doesn't want people to do it by accident.
South Park did an episode about drugs where, instead of using real drugs, the drug-of-choice was having a cat pee in your face. One wonders if anyone in the real world attempted to use this technique, with hilarious failure (Though it should be noted there are other episodes where they just use real drugs). Trey and Matt were making fun of the panic over kids trying to get high on things that wouldn't be considered obviously dangerous (like cough syrup, prescription medication, or aerosol spray).
In one episode of King of the Hill, Bobby gets a Love Interest (voiced by Lucy Liu) who tricks him into building a meth lab for his science fair project. All of the ingredients are explicitly shown, but the procedure is different than real meth preparation.
In Louisville, Kentucky, they put up huge billboards indicating ingredients used to make methamphetamine. One of which was lithium rechargeable batteries. The billboards say something along the lines of "They use this to make meth. Report suspicious activity to the LMPD (Louisville Metro Police Department) immediately."
Somewhat averted for the most part in Real Life. Many people that have bomb making experience don't mind telling you the ingredients. What they generally won't explain is how to do it safely, or how to build a detonator.