I’m fond of EJoyce’s metric of hammers and light bulbs (see that thread for more; they borrowed this metric from a puzzle hunt they did, and it’s in response to me proposing a couple types of puzzles…everything is connected).
They suggested that you can think of puzzle difficulty both in terms of “light bulbs” (how much of a leap of logic does it take to discover the solution?) and “hammers” (how much straightforward effort does it take to implement the solution?). Different people have different tastes on this, so many games have a mix of both: exploring and mapping until you find the thing you need would be a hammer-heavy puzzle, while realizing that thing can be used to bypass an obstacle could require more light bulbs.
My philosophy is that adding more hammers is easy but adding more light bulbs is difficult, so my advice will focus on the light bulbs. (You can always make your map bigger, for example, if you need more hammers, or break your key item into two pieces and hide them in different places.)
I think it was Dan Fabulich who said that the best light bulb puzzles, like the best jokes, are obvious in hindsight. Once you’ve realized the answer, it’s clear that it has to be the answer, even before you type it into the game. For that to happen, the player needs to have a clear understanding of what they’re capable of in the game—what actions are possible and what those actions will generally do.
My favorite way to get this clarity is systematicity. The player is able to do some unusual thing, and that thing follows predictable rules that you can try everywhere and learn via trial and error. In Scroll Thief, there’s a summoning spell that will teleport any living person to your location, but it requires line of sight. How can you use this to your advantage? Well, perhaps there’s an object that can’t be carried through a certain door, or an alarm will go off. Have your friend pick up the object, then look at them through the door, and summon them to you. Everything here follows clear and learnable rules that work throughout the entire game (you can summon a person with line of sight, you can see through open doors, you can tell someone to pick up an object and they’ll do it); it’s the combination of those regular rules that creates the light bulb puzzle.
Sometimes you don’t need this kind of full systematicity. Sometimes you can rely on players’ existing knowledge of the real world. If there’s a piece of cheese and a mouse that needs to be lured out of hiding, it doesn’t matter that nothing else in the game gets lured in by cheese—that’s a cliché about mice in the real world that many people will know. That can also make a good light bulb puzzle; just make sure the knowledge needed is really universal and not specific to your culture. (I am irrationally annoyed by the segment in Jigsaw where the actions you need to perform are blatantly obvious, but only if you’re familiar with the works of the 20th-century French author Marcel Proust. You’re given some objects that appear in his books and need to take the actions that the character in the story took.)
To close out, a few of my favorite types of puzzles that tend toward the light bulb side:
- There’s a passage that doesn’t allow you to bring object X through, but you need object X on the other side. I think it was zarf who said this was his favorite kind of classic text adventure puzzle, since it requires you to rethink your objects or rethink the map.
- There’s something (like a revolving door) that switches parity every time you take a certain action (like walk through it), and you need to find a way to break that parity.
- There’s an object that you used for its obvious purpose earlier in the game, but now you need to reuse it in a different way. The player is certain to know about the object already; they just have to connect it to their new problem.
- Early in the game, you see something that you have no way to interact with (like an aluminum wall), so it seems to just be decorative. Later, you gain a way of interacting with that thing (like an aluminum-cutting tool that can slice through any aluminum), so the earlier object takes on new meaning.