Enlarge ImageBoxed in. Nicotine (center) nestled into brain receptor "box." Credit: Dennis Dougherty If nicotine liked muscle receptors as much as it likes brain receptors, a single cigarette would kill. Scientists have finally figured out why the molecule is so picky--a finding that may shed light on the addictiveness of smoking. For nicotine--or any molecule--to interact with its receptor, the two must bind. Having opposite charges on the molecule and the receptor's binding site, referred to as the "box," helps. But the nicotine receptors in the brain and muscles are nearly identical--nicotine has a positive charge, and both receptors'...