Asimov's sf magazine has done well in its first two years--but not, judging from these cullings, on the basis of pioneering literary ambitions. The best stories here are for the most part clever, polished, efficient run-throughs of medium-interesting ideas: John Varley's ""Good-Bye, Robinson Crusoe,"" with its ingenious depiction of an artificial ""second childhood"" in an artificial paradise; the portrayals of an unwilling immortal in Sally Sellers' ""Perchance to Dream"" and the megalomaniac would-be-immortal in Garry R. Osgood's ""To Sin Against Systems""; the commandeering of human children by an occupying army of aliens in Kevin O'Donnell, Jr.'s ""Low Grade Ore."" Only a couple pack any memorable wallop: the horrific temporal-hijacking operation in Herb Boehm's stunning ""Air Raid"" and Brian Aldiss' elegant confrontation between a Chinese sage and a trans-human intelligence. There is an over-generous sampling of puns and gimmicks, along with a few brain-teasers from Martin Gardner and a pleasant but not especially illuminating essay by Jack Williamson on the teaching of science fiction.