Whenever I lose my watch, I take my sweet time to get a new one. I savor the freedom from my compulsion to carve my days into minute-size fragments. But my liberty has its limits. Even if I get rid of the clock strapped to my wrist, I cannot escape the one in my head. The human brain keeps time, from the flicker of milliseconds to the languorous unfurling of hours and days and years. It's the product of hundreds of millions of years of evolution. Keeping track of time is essential for perceiving what's happening around us and responding to it. In order to tell where a voice is coming from, we time how long it takes for the sound to reach both ears. And when we respond to the voice by speaking ourselves, we need precise timing to make ourselves understood. Our muscles in the mouth, tongue, and throat must all twitch in carefully timed choreography. It's just a brief pause that makes the difference between "Excuse me while I kiss the sky" and "Excuse me while I kiss this guy." Scientists are finding that telling time is also important to animals. At the University of Edinburgh, researchers built fake flowers with sugar inside to reveal how hummingbirds tell time. After hummingbird's drink nectar from real flowers, it takes time for the flowers to replenish their supply. The Scottish researchers refilled some of their fake flowers every 10 minutes and others every 20. Hummingbirds quickly learned just how long they had to wait before coming back to each kind. Scientists at the University of Georgia have discovered that rats do an excellent job of telling time too. They can be conditioned to wait two days after a meal to poke their noses into a trough and be rewarded with food. For 40 years, psychologists thought that humans and animals kept time with a biological version of a stopwatch. Somewhere in the brain, a regular series of pulses was being generated. When the brain needed to time some event, a gate opened and the pulses moved into some kind of counting device. One reason this clock model was so compelling: psychologists could use it to explain how our perception of time changes. Think about how your feeling of time slows down as you see a car crash on the road ahead, how it speeds up when you're wheeling around a dance floor in love. Psychologists argued that these experiences tweaked the pulse generator, speeding up the flow of pulses or slowing it down. But the fact is that the biology of the brain just doesn't work like the clocks we're familiar with. Neurons can do a good job of producing a steady series of pulses. They don't have what it takes to count pulses accurately for seconds or minutes or more. The mistakes we make in telling time also raise doubts about the clock models. If our brains really did work that way, we ought to do a better job of estimating long periods of time than short ones. Any individual pulse from the hypothetical clock would be a little bit slow or fast. Over a short time, the brain would accumulate just a few pulses, and so the error could be significant. The many pulses that pile up over long stretches of time should cancel their errors out. Unfortunately, that's not the case. As we estimate longer stretches of time, the range of errors gets bigger as well.