Counter Intuitive Growth

I realized something tonight for the first time. I mean, I've heard the theory but I think I just got it tonight.
I was realizing that even though I've been doing a lot more than usual the past few days, I feel like (given that I keep things reasonably balanced) I have energy to continue doing things. It seems counter intuitive to me that spending energy would create more energy, but it seems to be working. I've heard the concept before, mostly in regards to a fitness program giving you more energy. Apparently, it's true!
And then it occurred to me that the theory could be applied to a lot of things. Spending love creates more love, sharing happiness creates more happiness... Heh, it's just like a magic penny... (cue childhood memories montage).
I've heard this stated another way, too: "What you focus on, grows." I've seen that to be true in terms of thought processes, but it never occurred to me that it would have a more outward application.
So there ya go, my learning for the day.