Professor Lindstrom was waiting for us in his laboratory, in carpet slippers and without his tie. "Laboratory" is a perfectly silly term. The "apparatus" in any Psi lab is no more complicated than a folding screen, some playing cards, perhaps a deck of Rhine ESP cards and a slide rule. This place went so far as to sport a laboratory bench and a number of lab stools, on which Lindstrom, Mary Hall and I perched. My egghead Psi expert was barely able to restrain himself--he had some bitter things to tell me. I beat him to it. "Take that injured glower off your puss," I snapped. "Your business is testing people for their Psi powers. Why shouldn't I call on you for help? What are friends for?" "For a friend I might," Lindstrom said. "You don't rate that well with me any more." "I'll try to bear up under it," I told him. "In the meantime, this is Mary Hall, a reputed Psi. Her power is HC." He was interested in spite of himself. "Hallucination?" he said. "We don't see much of that, Miss Hall. And you claim you can demonstrate this power under controlled conditions?" These eggheads all talk alike. Mary shook her head. "No, I certainly do not. I'm as Normal as you are, Professor." He sagged slightly in disappointment. "Well," Lindstrom said. "This is going to be difficult to prove, Miss Hall. Merely by withholding your HC ability, you can act Normal--but what would that prove?" She turned to me. "I thought you said you had a way to get me off the hook," she protested. "How are we--?" "Quiet," I told her. "I didn't come up here for a lecture in logic. Especially from a dumb blonde." She started to bristle, but thought better of it. "It goes like this, Prof," I said. "This innocent looking piece of fluff was caught slipping a five-dollar bill to a teller at a bank down town, and asking for change for a hundred dollar bill. She says it was nothing more than sleight of hand. You are an experienced observer. I want you to watch her work her little trick. If she can fool us, and not use Psi, the legal position is that she didn't need Psi to fool the teller." I turned to her. "And the logical principle, Miss Aristotle," I told her, "is equally simple: Occam's Razor. Prefer the simpler explanation. Can you show us how you palmed the hundred and slipped the teller a five?" "You'll be watching for it," Mary protested, letting those ripe lips pout. "I suppose the teller wasn't? It's his business to watch the bills when he's making change." I took out my wallet and handed her a one and a five. "Hand me the one and make me think it's the five," I said. Lindstrom leaned his elbows on the black composition top of the lab bench, watching her narrowly. Mary got down off her stool and came over closer to me, smoothing the two bills in her fingers. The five was on top. "I'd like change for a five," she said, handing it to me. She worked it three times while we watched. "Utterly smooth," Lindstrom said. "I didn't see her make the switch."