An hourglass is a device that uses the passage of an amount of sand from the top to bottom half of the device to mark time.
The amount of time an hourglass can mark depends on the size of the hourglass and the amount of sand in it. An hour would be the standard amount, but hourglasses can also be bought that are egg timers for cooking hard or soft boiled eggs, or shower timers when people want to save water.
A stopwatch is a special type of watch that counts the passing of time rather than telling you what time it is. It has buttons to start, stop, and zero the count. It is used to time races and competitions.
Sundials are a device that uses the sun to tell the time. A sundial is made up of a horizontal dial plate with numbers on it, and a vertical upright called a gnomon. As the sun moves, the shadow cast by the gnomon moves around the dial plate, and you can read the time off what numbers it points to.
Some sundials use a shaft of light rather than a shadow, and the dial plate may be curved not flat. Some sundials have enough information on the dial plate to be able to tell the date as well as the time.
A watch is something you can use to tell the time on. Watches are small and usually have a strap on, so you can wear them around your wrist. Large watches that you hang on the wall are called clocks.
It is useful to know the time, so you know when to go to school, or have dinner, or watch your favorite TV show.
