A rock tied to a string that can swing freely is called a pendulum, and it turns out that one second is the time it takes any pendulum on Earth—regardless of weight—to swing from one end to the other, as long as the pendulum is 99.4cm long. This cool property of pendulums—that they always take just about the same amount of time to complete a swing no matter how far back you pull the weight before letting it drop—makes this a particularly easy experiment to perform. This property was discovered by a guy named Galileo Galilei in 1602, but you’re going to take the credit. It’s your property now.
