I learned about in interesting concept today: The Eruv. In a nutshell, Jewish beliefs prohibit certain activities on the Sabbath, such as moving an item from one domain to another. This presents certain problems, with some Orthodox Jews believing that things like a wheelchair are prohibited items that can not be moved. Luckily, the concept of a "domain" is broadly defined and some clever Jews have found a loophole: put a fence around the whole neighbourhood/city and declare it to be one big domain. The fence can even be a piece of fishing line at the top of a tall pole, so that it is basically invisible.
What I find ironic about this is how much of a technicality this is: God decrees that you can't do something, but doesn't specify all the details, so Men stretch the details as far as they can. In all fairness though, opinions are split in the Jewish community about how large an eruv can be, and whether it makes sense for one to be the size of a neighbourhood. My feeling is that it's rather silly to make an eruv that large, since that basically neuters the entire Law (or does it spay it? Is the Law male of female?).
I read somewhere an interesting idea for those who believe an eruv can be as large as they want: make one eruv for the whole earth. The Earth is a sphere, so by definition any enclosed shape on the surface of the sphere actually just divides the sphere into two surfaces. It's then merely a matter of convention about which side of the line is the inside. Thus one eruv would serve the whole world. Somehow I doubt they'll go for it though.