Affordance is an invented word  that has a history of redefinition, so it’s an appropriate first entry for this Writer’s Grimoire in which I’m going to redefine, repurpose or reframe words such that they serve as convenient handles for concepts that are useful for writers.
I’m familiar with affordance from the tech world, where it means something that allows a user to manipulate an object or interface. An example of a hardware affordance would be a doorknob that allows you to open a door or pull it shut behind you. It’s presence suggests that a particular bit of wall actually is a door, and its design informs you how to use it.  In software, scroll bars and clickable buttons would be affordances.
But how is that relevant to writers, unless you’re cranking out an article on industrial design or software user interface guidelines?
An affordance beckons. It invites interaction. It’s the physical (or digital) manifestation of a call to adventure.
The first time I ever used “affordance” in the context I’m describing here, Stephanie  and I were island hopping in Ha Long Bay. We’d just beached our kayak and were about to explore a cave system that was the main attraction when Stephanie noticed a set of concrete stairs that started below the waterline and crept up and around a rocky outcropping to who knows where.
“We have to take these stairs,” she said.
“Of course,” I said. “They’re an affordance.”
Meaning we must because we can. We were on an adventure, and we had to seize the opportunities presented to us. 
As a writer, you can lay an affordance in your hero’s path to steer them towards your story—the green door that leads to a secret garden, a trail of breadcrumbs that mark a path deeper into the woods, the torn corner of a photo that hides an older photo beneath.
Affordances also signal to the reader what’s possible in the world you’ve created. They can help define or illustrate your character’s capabilities. A flagpole that serves as a handy anchor point for Spider-Man to swing from one building to another would be useless to Aunt May.  Once those capabilities are well understood, you can expand or restrict the availability of affordances to steer or limit your character.
Beware of false affordances. If something looks like a step, people will step on it, which is why you see “Not A Step” stickers on airplane wings and the paint tray at the top of ladders, and why the first time you went up in the attic your dad warned you to tread only on the beams.
If you offer a false affordance, your readers will be wondering of your main character, “But why don’t they just …?” You may have a reason, but you have to make it clear that that escape hatch is welded shut. 
Next time: 2020 Year in Review (ugh)
 All words are invented, but affordance was coined relatively recently, in 1977 by James J. Gibson in his article, “The Theory of Affordances.” Gibson’s definition is fairly broad and geared toward ecology, but it was reimagined (and popularized) by Don Norman, author of The Design of Everyday Things, to apply specifically to usability design.
 Hopefully. But we’ve all seen people try to pull open a door that needs to be pushed because the handle design is the same on both sides.
 Frequent creative collaborator and epic best friend, Stephanie Keiko Kong.
 The stairs didn’t lead anywhere particularly interesting—I think they were the remains of an old pier—but we had a unique mini-adventure that none of the other tourists on the island experienced.
 More on this in a future entry on Asymmetry.
 Unless you want to exploit that false affordance for dramatic or comic effect—insert montage of characters crashing through air vents and ceiling tiles.