It seems easy to define "Monday": some day is a Monday if and only if it comes immediately after a Sunday. The problem is that if we do the same for every day of the week, our definitions will become circular at the seventh try. The only way I can see out of this is to say, for instance, that May 18, 2009, is a Monday, or that May 18, 1750, was a Monday (according to the Gregorian calendar), or that today is Monday. But isn't it strange that we have to give an example in our definition? And are there other words that we can only define with an example?