Thousands of years before mechanical timepieces existed, as far back as the 16th century BCE, water clocks were among the most accurate ways to tell time. These early clocks controlled the flow of water from one container to another in order to measure the passage of time with remarkable precision — and unlike sundials, they could be used at night. Civilizations all over the world used water clocks, but the ancient Greeks in particular were known for improving the mechanism with a timepiece they called “klepsydra,” or “water thief.” 

The philosopher Plato is credited with one particularly ahead-of-its-time innovation: In the fourth century BCE, he built a set of klepsydra alarm clocks meant to rouse the students at his academy. The clocks had two basins, one emptying into the other, and would run throughout the night. When the second basin filled with water in the morning, it would trigger a sound. One of the clocks had a container that made a whistling sound when air was forced out of it. Another had pebbles rigged over the second tank that would fall and rattle when it was full. Aristotle was a student at Plato’s academy, so it’s possible he was roused by these very clocks. In the third century BCE, Greek inventor Ctesibius of Alexandria took the design a step further. He added a top tank with an overflow valve that allowed a lower tank to rise to keep time, making little noises on the way up like a cuckoo clock.

Before it was possible to have just about any item conceivable delivered to your door with the click of a button, getting the mail took much more effort. When the United States was founded, people had to travel to pick up their mail at a post office, a trip that, for some, could take a full day. By the early 1900s, mail was being delivered to both urban and rural homes by dutiful postal workers, but as communities embraced the new services, new challenges emerged. Rural residents repurposed old food containers and empty oil drums — sometimes still coated in residue — as mailboxes. City residents, meanwhile, didn’t always use receptacles, instead relying on the established routine of letter carriers making deliveries straight to their door; the postal workers would knock first, and wait for a response before handing the mail over. 

None of these solutions, however, was terribly convenient for the postal workers who delivered the mail. The Post Office Department (now the U.S. Postal Service) worked to set standards for mailboxes and encourage their use, and in 1923, it was officially mandated that every household have a mailbox or a letter slot. Certain criteria were put in place for mailboxes: They should be affordable but durable; they had to be easy to use, but also secure against theft; their size needed to accommodate a variety of deliverables; and they should have a flag or something similar to indicate mail was ready for collection. Postal workers benefited tangibly from the changes. Not only were the cleanliness and consistency of a mailbox more easily guaranteed, but valuable time was reclaimed. The Post Office estimated that mail carriers who were still knocking on doors lost approximately an hour and a half each day just waiting for someone to answer.