The world's most precise clocks run at a steady pace, messing up by only about 1 second every 300 million years. But the brain takes those rhythmic seconds and makes its own sense of time — stretching the ticks and scrunching the tocks. But why can't the brain keep time like a regular clock? In other words, why does time fly when you're having fun, and why does it plod along when you're bored? How the brain percieves time depends on its expectations. The brain can represent the probability that something is going to occur, given that it hasn't happened yet, said Dr. Michael Shadlen, a neuroscientist at Columbia University Irving Medical Center in New York City.