"It isn't too hard to believe that certain things can happen to Time on the wrong side of yesterday--or tomorrow! Time--the physicists tell us--never stops flowing. It's like a melting candle or silk before it hardens on the loom--all crinkled up and sparkling like a dew-drenched spider web. "If Time melts in a back-of-yesterday dimension what's to stop a man from dissolving with it, and running in a thin trickle back to his yesterdays? You were a boy once and you could be a boy again--without ceasing to be a man. "Put it this way. On the dark side of the moon there was a valley of shadows. A big, blundering fool went stumbling into it, and landed in a heap. Before he could pick himself up a part of himself dissolved in some unimaginable backwash of time, and he became a boy again. His boy-self split off from him, and went stumbling off over the plain in a suit five sizes too large for him. "It's not as impossible as it sounds. The boy you were still exists in Time, and he could emerge from the past to stand beside you in a vortex of dissolving Time. Was there something in the valley that could change the flow of time, reverse it, and twist it around like butter in a churn? "The answer was right there in the cave with me. But I couldn't see it because _another_ space-suited figure was making my brain whirl. He'd come clumping into the cave bent nearly double, and now he was shuffling toward me as though I'd committed some horrible crime I could never hope to atone for. "Through the pane of his helmet his eyes burned accusingly into mine. But it wasn't until he halted directly in front of me and lifted the helmet from his head that I knew what my crime was and why he found it hard to forgive me. "I had committed the crime of living beyond my alloted span! The man facing me was old ... old. His face was still my face, but if ever I had been young and handsome and a target for the wiles of a pretty woman I was so no longer! "He seemed to realize that I could hardly bear to look upon myself as I would be, for he spoke sharply, quickly, without attempting to explain his presence, or even to prepare me for what he had to say by working up to it like a story-teller with a great load of unimaginable horror on his mind. "'It's a monstrous beast of prey!' he croaked. 'It can dissolve Time and re-shape Time in a hundred horrible ways!' "He quirked his head at me. 'You know more than that lad but I know more than you--for I have lived through this moment before! Once long ago I stood in this cave and warned you! You are at the crossroads of a branching future! If you take the right turn now you will live to become me. But, if you take the wrong turn--' "He straightened, and pointed with his gloved forefinger into the shadows behind me. 'It is there--at your back! When you turn you will see the shining web which it uses to dissolve Time! All over this valley the creature has thrown a Time-dissolving web of force!'