You bought a watch that tells the correct time 90% of the time you look at it. The other 10% of time it might give you an obviously incorrect time (say, showing 11PM during the day) or might be just slightly off to make you miss that important meeting. After using this watch for a while you notice that its correctness depends on several factors such as the time of the day, your location in longitude (but not latitude), your viewing angle at the watch face, what you had for breakfast the previous day, and so on. When you need to find out if you can catch the next bus, for example, you learn to first move to a location with the optimal ambient lighting, tilt your head just so, and raise your wrist at just the right speed to ensure a correct reading. You went back to the store to exchange the watch for a better one that is correct 95% of the time. However, it comes with a different set of quirks for getting the correct time. You need to remember to rub your right cheek before reading