Even more than projectile motion, our understanding is bedeviled by the arrow of time. So many things seem obvious in retrospect, and yet they arrive like thunderbolts in the way that only a new piece of obvious information can. Okay, I guess a thunderbolt can arrive that way, too, since that is how similes work. In retrospect, it was obvious. Constantly, we are not so much learning things as realizing them, and once they become real to us they cannot be un-known. Today is Friday, briefly coalesced from the unknowable future on its way to the rigid past. Won’t you realize some mind-blowing stuff with me?