Why are manhole covers round?  I heard this a lot when I first started at Microsoft, less and less as the years went by, but never really cared to know the true answer.  Now, via digg, there's a whole (ha!) explanation on wikipedia of origins as an interview question and the "answer".

What's even more interesting is the conversation generated in the comments of the digg post.  From criticisms of Microsoft to theories on why manhole covers are actually round; 235 of them in fact. 

Just goes to show that there are a whole lotta ways to approach and answer these types of questions.  You should check it out.