[xeonmc]

When being "here" and "there" is exactly the same, you move spontaneously.

When being "here" and "there" is almost-but-not-quite the same, you move easily.

The degree of "not-the-same-ness" is called the Lagrangian.

In other words, symmetry is "fungibility of states".

Things happen because the before and after is not very different, the transaction of different-ness is the energy involved.

In a classical system it's nigh impossible to encounter an "exactly the same" situation, because there are just too damn many participants to rule out every possible interaction.

In a quantum system you encounter "exactly the same" situations frequently because there is only a tiny number of participants interacting.