Smallest System

I remember being interrogated by an old family friend and former babysitter. This eminent professor of the dark arts asked me, "Why is hydrogen so important for our entire discipline?"

At that time, I had no idea, so I blathered on for a while.

Eventually, he got frustrated enough to say this: "Because it's the simplest atom. Just one proton and one electron."

And thus did the young man of three decades gone become the old man of today.