Hourglass

Okay, so an hourglass is a special type of timer that has two parts connected by a thin passage or channel. The top part is filled with tiny grains of sand (like the sand you find at the beach) and the bottom part is also empty.

When you want to start the timer, you turn the hourglass upside down. The sand will start pouring from the top part into the bottom part, passing through the thin passage. As the sand flows down, it slowly fills up the bottom part.

The reason it takes time is because the sand particles are really small and have to squeeze through the tiny channel. The longer the channel, the longer the sand will take to pass through it. And the more sand there is in the top part, the longer it will take for it to pour down completely.

Most hourglasses are designed to measure about an hour (hence the name), but they can also measure shorter amounts of time. When all the sand has passed through the thin passage and filled up the bottom part, the timer is up and you know that your time is up too!