"Poka yoke" is a Japanese phrase meaning mistake proofing. It is meant to shape behavior at the level of execution. There are two approaches to this. One is to prevent humans from making a mistake, to physically prevent it. If it's not possible to prevent the mistake, you can use poka yoke to anticipate and warn of mistakes. Perhaps the easiest way to understand poka yoke is to see some examples. There are many applications of mistake proofing, many of them are related to safety. The microwave oven will not operate if the door is open, rumble strips on a highway let you know if you're wandering off the road, and speed bumps cause you to check your speed. If you've ever painted a white ceiling you know that it's extremely difficult to see if you've missed any spots because you're painting white over white. Later, when the light changes, you can see your mistakes. Glidden makes a ceiling paint that goes on pink, making it easy to see if you've covered everything; then when it dries, it turns white. You know why manhole covers are round? That's the only shape that cannot possibly fall in the hole. Most sinks have small slots or holes at the front or the back near the top. Their purpose is to prevent overflow if you leave the water on. In developed countries many restrooms have motion-sensitive lights in commodes. The lights are not on except when needed, saving energy, and commodes flush automatically. In manufacturing most machines are well-guarded these days. Injection molding machines, for example, have mechanical, hydraulic, and electrical interlocks that all engage to prevent the machine from working when you slide open the door. This is a novel poka yoke to ensure that bolts are tight. There's a washer with bits of plastic on it that goes under the head of the bolt. When the bolt is properly tightened, the plastic squirts out. This is a common jack for phone or internet. The tab marked B keeps the connector from being inserted upside down. The tab marked A is designed to protect tab B and keep it from being broken off. Knowledge in the world is information that's embedded, encoded, or implicitly resides in the components of a task. Here, the shape of the dipsticks communicates their purpose. The yellow dipstick is shaped like an O and is used to check the oil level. The orange dipstick is shaped like a T and is used to check, you guessed it, transmission fluid level. It's ironic that this design is very language-specific but comes on a Japanese brand. New lawnmowers are required to have a safety bar on the handle that must be pulled back in order to start the engine. If you let go of the safety bar the mower blade stops in three seconds or less. This is a medical example. A tube is inserted into the patient's lungs. But how do you know if it's in the right place, in the lungs, not the stomach? Squeeze the plastic bulb and then put it on the tube. If the bulb inflates with air, the tube is in the lungs. If not, then an error has occurred. This final example has three poka yokes designed in. In countries where they drive on the left side of the road visitors can get confused. Offset crosswalks and accompanying railings force pedestrians to turn and walk toward oncoming traffic so they can see what's coming. Instructions printed on the blacktop or curb say "look left" to direct pedestrian vision in the proper direction and the jagged lines warn drivers that they're approaching a crossing.