When people think about growing old, they blame the passage of time—the years roll by, and the body stops looking younger year by year. But the latest science disputes this view. A person ages because the cells in their body age, and cells live only in the present. This is one reason memory remains such a mystery. Brain cells function through electrochemical activity that occurs the instant a chemical reaction or electrical impulse is able to occur. There are no pauses to think about reacting; if the potential is there, the action must follow.