An hourglass is a device that measures time by letting sand flow from one glass bulb to another through a narrow opening in the middle. Picture two round glass chambers connected by a thin neck, like the number 8. When you flip it over, gravity pulls the sand down grain by grain at a steady rate. When all the sand has fallen to the bottom chamber, you know a specific amount of time has passed.

Before clocks became common, people used hourglasses to time everything from cooking to church sermons to ships' watches at sea. Sailors depended on them to keep track of time on the ocean. The classic hourglass takes exactly one hour for the sand to fall through, which is how it got its name, but people made them in many sizes: some measured just a few minutes (useful for timing a soft-boiled egg), while others measured several hours.