Invisible Questions

The electrical grid is made up of masts of different shapes that carry a variable number of power lines. What is the logic behind this variety?

Mountains are generally considered terrain elevations. But what distinguishes a mountain from a hill? And if an elevation is very close to the one next to it, when do they still belong to the same mountain, and when do they become mountains in their own right?

Some hard cheeses are full of holes, commonly known as eyes. Where do these eyes come from, and why did they start to disappear in what has come to be known as the Great Eye Crisis?

Most fish are vertical, but some are horizontal. How come?