The joke goes, "Time flies like an arrow; fruit flies like a banana." Time theoretically runs at the same pace for every living species on this planet. But time can mean a lot of things to a lot of people. Our bodies align with time—how many heartbeats per minute, how much time we spend sleeping, how many days until a woman's next menstrual cycle. When you're younger you have all the time in the world. And when you're older you might feel there's not enough time. Time, though often used to measure durations, can also measure how we live our lives.