So, why are manhole covers round?

[youtube=http://www.youtube.com/watch?v=njZ0Dnu7aEc&w=500&h=315]

I'm not all about turning this blog into the same thing as Wikipedia -- because you can find the basic answer to the question posed above by searching 'manhole cover' on there -- but I do want to use it periodically to address broader questions that no one really thinks about until it's rig…