When we do two things at the same time, our brain divides the work in half, literally: each hemisphere concentrates on one task, reports a study in the April 16 issue of Science. Researchers measured brain activity in volunteers performing letter-pairing tests. When subjects had to deal with two streams of letters, concurrently performing two pairing tasks, the activity in one half of the brain corresponded to one task, and the activity in the other half corresponded to the other task. The study might explain why people are notoriously poor at doing three or more things simultaneously. After two tasks, we run out of hemispheres.