I came across the 5 Whys (via Jason Kottke), and was extremely pleased to find it as a technique used in Toyota. For quite sometime I have been consciously trying to ask a series of Whys during designing to arrive at either a business decision or a fact. It saves from making assumptions about hidden and underlying purposes. Any form of problem solving approach, including designing, will have to do this one way or the other. The only way I could explain it to others was through the analogy of a ladder. I get down the ladder step by step with every why and finally get down on the base of it; and you can imagine that it was not very effective. An established technique like 5 Whys does a much better job than that, and am I glad to discover it!