You say things like this every day, all the time. But there was a time when time itself was undefined--no one knew the difference between a minute, an hour, or a day.
Then people started creating tools to measure time. First they used the big stuff around them--the sun, the moon, water. Soon after, using the knowledge they got from their natural time-telling tools, people began to build clocks--huge clocks unlike the ones we use today. They also used their knowledge of the sun and moon to create calendars made up of months and years.
Now, centuries later, we have clocks all around us. We can easily figure out how long a month is. But it took many years of tinkering and inventing to perfect the art of telling time. You could take a few moments now to read all about time. If you have a minute, that is. This title has been selected as a Common Core Text Exemplar (Grades 4-5, Informational Texts)