It's not exactly one of the 7 wonders of the world but its something I have on occasion wondered about. Why does making an optical fibre thin make it  only possible for one mode to be propagated  through it. If you haven't done the communications option recently, and let's face it who has, then I might have lost you. The modes in an optical fibre are the different paths that light can take, if you look at the left hand image you can see two different modes. Anyway, the thing that confused me was that even though a fibre is thin you can still get the light to travel along it bouncing off the sides at different angles. What I normally do in situations like this is I turn to a simulation like Algodoo, it is great for optics as well as mechanics. I soon discovered that I was right, it is possible to send more than one mode down a thin fibre, however if you consider the light coming from some point source then you'd have to get it very close to the end of the fibre to get all but the light parallel to the fibre to enter. Another mystery solved.