My son is now smack in the middle of the "Why?" phase, made famous by Mindy from Animaniacs. He also asks "How that works?" about many things (the car, a fan, ...) The thing is, as annoying as these questions can be, I don't want to discourage him from asking. And I want to answer him as best as possible.
I admittedly have a tendency to answer the questions along the lines of "A dialogue with Sarah, aged 3: In which it is shown that if your dad is a chemistry professor, asking 'why' can be dangerous" by Stephen McNeil (read it!). But, clearly, a nearly-3 year old asking 'how the car works' isn't interested in the details of the internal combustion engine and advanced drive-by-wire technologies, and 'why he shouldn't play with sharp knives' (with 20 followup 'whys') doesn't really want to know all about blood flow and ER copays. So, it occurred to me that I should be scientific about this, and experiment to find out the type of answer he's looking for.
So, over the last few weeks I've been asking him "How that works?" and "Why?" and I've determined that these are two very different questions in their meaning and purpose.
"How that works?" can be translated to adult as, "How do you use that?" For example, when I asked him how LEGO bricks work, he proceeded to show me how they stack together and how you can make towers and tunnels, etc. So, now when he asks me about the fan, I show him how to use the switch. Or the car, I'll point out the steering wheel and pedals. This seems to be working quite well (he's not driving yet, though).
"Why?", on the other hand, is used purely as a stalling technique or because he doesn't want to listen. It seems that "Because," played after a few real (or fake) answers, is a fairly consistent final answer for reaching the desired "OK, I love you, bye bye" stage of the game. And, it turns out, it's just as annoying to a toddler as it is to an adult.