Clocks tell time and help people know when to wake up, go to sleep, attend meetings or games. They were invented in 1797 by Eli Terry and have changed over time, originally using Roman numerals instead of numbers. While clocks are useful for keeping on schedule, they can be inaccurate or take up space, though watches serve as a substitute. Clocks can cost anywhere from $1 to $500 and are sold at stores like Best Buy, Walmart, Target and more.