The 9th Annual Year in Ideas.

For the ninth consecutive December, The Times Magazine has chosen to look back on the past year from our favored perch: ideas. Like a magpie building its nest, we have hunted eclectically, though not without discrimination, for noteworthy notions of 2009 -- the twigs and sticks and shiny paper scraps of human ingenuity, which, when collected and woven together, form a sort ofcognitive shelter, in which the curious mind can incubate, hatch and feather. Unlike birds, we can also alphabetize. And so we hereby present, from A to Z, the most clever, important, silly and just plain weird innovations we carried back from all corners of the thinking world -- ideas from science and art, politics and policy, technology and engineering, zombie studies and . . . well, whatever you call it when someone tries to figure out whether a full or an empty beer bottle makes for a better blunt-force weapon. To offer a nonalphabetical option for navigating the entries, this year we have attached tags to each item -- in the print issue and online -- indicating subject matter. However you choose to fly, we hope you enjoy. [ABSTRACT FROM PUBLISHER]