Humans have used many devices to measure time. The sundial was one of the earliest and simplest. A sundial measures the movement of the sun across the sky each day. It has a stick or other object that rises above a flat surface. The stick, blocking sunlight, creates a shadow. As the sun moves, so does the shadow of the stick across the flat surface. Marks on the surface show the passing of hours, and perhaps, minutes. The sundial works well only when the sun is shining. So, other ways were invented to measure the passing of time. One device is the hourglass.The hourglass is shaped like the number eight --- wide at the top and bottom, but very thin in the middle. In a true "hour" glass, it takes exactly one hour for all the sand to drop from the top to the bottom through a very small opening in the middle. When the hourglass is turned with the upside down, it begins to mark the passing of another hour. By the eighteenth century, people had developed mechanical clocks and watches. And today, many of our clocks and watches are electronic.