A few years back, my three-year-old son Max had an unyielding passion for Thomas the Tank Engine trains. Piece by piece, he accumulated a rather impressive collection. But here is the thing that fascinated me as a parent: every time Max received a new train that he had obsessed about, which he just had to have, he promptly took out the catalog to identify the next train that he could no longer live without. So once he acquired Thomas, Fearless Freddie had to be next, then Clarabel, followed by Duncan, Rusty, Diesel 10, and so on. As parents, we naturally anticipated after each purchase that Max would finally consider his collection complete. But for Max, what was equally natural was to expect his train portfolio to continue to expand indefinitely, or at least until the enchantment ended. A big part of the thrill of building his collection in the first place was the possibility of its everlasting expansion and enhancement.