One of the classrooms in which I teach is adjacent to a chemistry lab. A solid wooden door connects the two rooms. Until recently, a large white sign with red lettering was posted on the door, visible to all in the classroom, reading, "Fire door. Do not block." I found nothing remarkable about this arrangement until one day I noticed that the door has no knob, no visible way of opening it. Further examination showed that the hinges are on the inside of the door, so that it opens into the classroom. A bit of thought led to the realization that the door is for the students in the chemistry lab; if a fire breaks out in the lab they can escape into the classroom and then out into the corridor and out of the building.

All well and good, but where does that leave students in the classroom? Imagine a fire alarm going off and the smell of smoke in the air. My students rush to what looks to be the most appropriate exit, and find that there's no way of opening the door marked "Fire door", and that pushing on it is not the solution in any case. When I describe this scenario to my students in the classroom, as an example of inadequate design in our immediate surroundings, it usually gets a few chuckles, despite the perhaps macabre context. Still, they can learn a few lessons about design from this example.

Messages are targeted at specific audiences, and messages must be appropriate for their audience. Here we have two potential audiences, the students in each of the two adjoining rooms. For the students in the chemistry lab, the sign would be perfectly appropriate if it were visible on the other side of the door. For the students in the classroom, less information would actually improve the message: "Important: Do not block this door" would be sufficient. This avoids drawing attention to the function of the door, functionality that is not targeted at those reading the sign. In general, conveying an unambiguous message can be difficult and requires careful thought.

The sign no longer hangs on the door, which now stands blank.