Answer: Candles

When you want to keep track of the passage of time, say, to return to your kitchen after an hour to check on dinner or to track a few hours of work before taking a break, then you most likely take an analog route by using a physical timer like an old mechanical egg timer, or go the digital route by pulling out your trusty smartphone to set a timer on it.

One analog route that you definitely don’t take, unless you’re the anachronistic sort of person bent on living life like it’s the 15th century, is to stick nails into the side of a candle. Yet that (rather clever if we do say so) way of keeping track of time was commonly used in Europe before the advent of inexpensive mechanical time pieces.

How does a candle clock work? Because the burn time of a standard size candle is consistent, the clock was simply a candle with a backing plate with marked segmentations of an hour (or longer), like the one seen here, or a candle with consistently spaced markings sitting on a metal plate. You stuck a nail into the candle at the time mark you wanted to be alerted at and, when the candle burned to that point and the wax softened, the nail would fall out and clatter loudly on the metal tray below.