In his wonderful book The Evolution of Useful Things, the author and civil engineer Henry Petroski shares one of the most famous case studies in the history of design:

Before it was destroyed by fire, L’Hotel Louis XIV, which was located on the waterfront in Quebec, advertised private baths. However, their privacy was of a limited and precarious kind, for each bath was located between a pair of guest rooms, both of which opened into it. This arrangement is not uncommon in private homes, where bedrooms share a bathroom or where a bathroom opening onto a bedroom also opens into a hallway. In all such situations, the basic design objective is to achieve privacy for whoever might be using the bathroom. This can be achieved in many ways, of course, and the most obvious and common way is to have locks on each of the doors, so that the bathroom user may bar others from entering. The failure of this solution is frequent and frustrating: the person who has finished with the bathroom forgets to unlock the second door, causing at least a little inconvenience for the next user who tries to enter it. In bathrooms shared by siblings, screaming through the locked door may or may not get results, but generally there is little more than the temporary inconvenience of having to go around to the other door or to another bathroom in the house. Families that find bathroom doors too frequently locked can remove all locks from the doors and trust everyone to knock before entering.

“In the case of bathrooms shared by unrelated guests,” Petroski continues, “the problem is less easily solved.” Consider, for example, the hotel bathroom shown above. There are two doors, each opening into a separate room occupied by a guest who is presumably a stranger to the other. You need locks on both doors to ensure privacy for each occupant, which means that you’ll invariably wind up with situations in which one guest leaves and forgets to unlock the second door, leading to considerable inconvenience. What do you do? You could, of course, tear down and rebuild the entire hotel, at great expense, so that each room has its own bathroom—a solution that might sound ridiculous, but isn’t so far removed from how similar design problems are addressed every day. More plausibly, you could somehow label the doors. Petroski notes that this was the approach employed by a similar house in which he once stayed in St. Louis: “The measures taken to avoid this situation consisted of a nicely printed sign placed prominently on the dresser beside the bathroom door, reminding each guest to unlock the other guest’s door before leaving the bathroom. I am sure I was not the only guest who suffered from the inadequacy of that solution.” Alternatively, if you were of a mechanical disposition, you could rig up an alarm system that would sound a buzzer if one door was unlocked without the other. This would have obvious shortcomings in practice, and it would also annoy guests who just wanted to use the bathroom in peace.

In the end, the proprietors of the Hotel Louis XIV came up with an ingenious answer, as outlined in the book By Design by Ralph Caplan, which was Petroski’s source for the original case study:

Well, there were no locks on the bathroom doors of the Louis XIV, but tied to each doorknob was a three-and-a-half foot length of leather thong to which a hook was attached. When you were in the bathroom you simply linked the two hooks together, holding both doors shut. There was no way to get back into your own room without it at the same time unlocking the door for the other guest. It was memorable as the total integration of object and circumstance.

It’s a lovely solution—so much so, in fact, that Caplan’s book puts it into its own subtitle: Why There Are No Locks on the Bathroom Doors in the Hotel Louis XIV. And at the heart of the answer lies a subtle change in the way the problem is understood. Other measures, like putting up a sign, focused on the idea that both doors had to have locks, when the lock is really just an interim solution to the underlying problem, rather than the problem itself. As Petroski puts it: “The basic design objective is to achieve privacy for whoever might be using the bathroom.” And once the problem is phrased in such a way as to leave locks out of the equation entirely, you’re that much closer to figuring out how to address it.

And it’s a lesson that writers or other creative artists should take to heart. In On Directing Film, David Mamet devotes what seems like an inordinate amount of time to teaching his students how to properly describe the objective of a scene, and reminding them how easy it is to confuse the means with the end. One of the pitfalls of figuring out a plot is that once we’ve come up with a “solution” to a problem, like putting locks on the doors, we spend all our energy trying to get out of all the new complications that the solution presents, rather than focusing on the issue that it was meant to satisfy. In many cases, like the proprietors of the rooming house in St. Louis, we end up affixing a label to explain what we mean, which is as close as you can get to unambiguous evidence that the solution you have in mind isn’t working. Donald Norman, in his classic book The Design of Everyday Things, provides examples of the labels we see on everything from doors to to hot and cold water faucets, and concludes: “When simple things need instructions, it is a certain sign of poor design.” And that’s as true of stories as of sinks. Whenever a movie gives us an introductory scroll of text, a lingering shot of a sign, or a chyron to explain where we are or how much time has passed, it signals that the underlying problem hasn’t really been solved. If you want to find the right answer, you have to start by asking the right question. And then you can soak in the tub until you’ve figured it out—as long as you remember to lock the door first.