"The big question about violins is why do the good ones sound like they do and why do the crappy ones sound like they do," says Northwestern University ethnomusicologist Stephen Hill. Hill makes his living pondering questions such as "Why is it that all human societies have music?" He's also an amateur fiddler and woodworker, with a yen to build his own instrument. It's a daunting project, he says. Violins are made of 70 pieces and four or five woods, including dense, stable maple for the back and light, springy spruce on the front. You can learn the physics of it, but it's an intuitive process, with "truly brilliant" violins coming in more or less mysterious ways from "the hands of brilliant craftsmen." The standard line on Antonio Stradivari, for example, is that his secret lay in the varnish. "But chemical analyses have been done and the varnish has been reproduced, without the same results," says Hill. Another unproved theory is that a lot of the wood Stradivari bought was from a shipment that had sunk years before--"Lying in salt water, the pitch and sap inside the pores of the wood was dissolved away, so it was lighter and could respond faster to vibrations." Then again, Hill says, it might be a matter of aging: "For a fiddle to really sound good maybe it needs a hundred years or so." He'll discuss Fiddling Around: Using Wood in Instruments from 3 to 4:30 on Sunday, February 2, at the Morton Arboretum's Thornhill Education Center, 4100 Route 53 in Lisle. It's free, but parking is $7. Call 630-719-2465.