Time

"But sir, maybe if we could just travel through time and..." The wizard's chuckle cut off the boy's words. "My dear child, one cannot simply travel through time, as if it were some kind of spatial dimension." A desperate and puzzled look came to the boy's face. "Time is a discrete series of events. One thing happens, then another, then another, and so on. Once a thing has happened it cannot simply be reversed. It has happened. Action and reaction. Deed and consequence. Discrete moments trickling past more rapidly and silently than any can perceive. You live on the imperceptibly sharp edge of a galactic knife, endlessly slicing the past from the future. The real vine, the future, is alive, green and growing. The past, well, it's dead, son." A tear came to the boy's eye. "Ah, little one. When you see that place, you will wonder why you ever wept for this."