Bruce Koscielniak, in this Common Core text exemplar, tells the intriguing story of the many years spent tinkering and inventing to perfect the art of telling time. When time itself was undefined, no one knew the difference between a minute, an hour, and a day. Then people started creating tools to measure time. First they used the sun, the moon, and the water, but soon after people began using their knowledge about the natural world to build clocks and to create calendars made up of months and years. Centuries later, we have clocks and calendars all around us!