It hits you hard if you don't expect it and it usually leaves a mark, or at least a memory.
It's also what you use to build a monument (or raise a wall, for that matter).
On a personal note, I don't think there's a better feeling than the one you get when you say to yourself from the heart that "today is the day I'm changing something [about myself or my system]".
That's me this monday. A happy week to y'all!