Any day of the week you can look outside and see squirrels scampering across power lines without any apparent difficulty. They can go from line to tree, from line to house - no problem. Then, every once in a while, there comes a frightening flash of light and a deceased squirrel drops to the ground. What went wrong?
The answer lies in the way electricity travels. Unless it is blocked altogether by a non-conducting medium, an electrical current will always follow the path of least resistance between any two points. Power lines are sometimes insulated, but in many cases they are simply exposed to the air. That's safe enough because the air itself acts as an insulator: the electricity won't spark off into the space around lines. But since the lines are uninsulated that means that squirrels running on these lines are coming into direct contact with the flow of electricity.
Then why don't they get shocked every time they run on a wire? It's because the electricity would not get from one end of the wire to the other any faster by traveling through the squirrel; that would be a diversion from its path. It's only when it happens to bridge a gap with its body - say if it touches the transformer with a front foot while leaving a back foot on the wire - that the electricity will follow the easier path of conduction, this time right through the squirrel.
That tendency to follow the shorter path, by the way, is exactly what is meant by "short circuit" - the unfortunate squirrel has inadvertently acted as a short circuit for the power line. Lights dim, the squirrel drops, and the power resumes.