There are just so many things in physics that seem "elementary" because they never get explained, but I don't feel like "I could have invented them", so my brain refuses to see past them and understand stuff that relies on them. I still don't have satisfying answers to questions such as:

* What's the difference, in terms of intuition, between "momentum" and "energy"?
* Why is "generalized momentum" called "momentum"?
* What's the point of the principle of least action if you already know what state your system is going to end up in?
* What's the intuition behind "action" anyway?

Feel free to suggest answers to those questions, though I suspect there's no easy answer that would fully satisfy me. For instance, I know that momentum is a vector and energy is a scalar, so pointing out the fact that momentum is "directed" in sone way might be considered an answer to my first question, but even knowing that, I still feel like I'm missing something.