Science can finally explain why the dumbest TV commercial will draw us away from a really good novel. Attention, it turns out, is a finite commodity. "It takes a lot of your prefrontal brain power to force yourself not to process a strong input like a television commercial," says Dr. Robert Desimone, an MIT neuroscientist. "If you're trying to read a book at the same time, you may not have the resources left to focus on the words." The either/or nature of attention, says the writer of guidebook on concentration, exposes the myth of multitasking.