A day is the time it takes for Earth to spin once on its axis, which we experience as one cycle of daylight and darkness. This takes about 24 hours. When the sun rises in the morning, we say a new day has begun. When it sets at night, that day ends.

People use the word in different ways depending on context. When your teacher says “See you tomorrow,” she means the next day on the calendar. When someone talks about “back in my grandfather's day,” they mean the time period when he was young. A day off means a break from school or work.

The word also refers just to the bright hours when the sun is up, as opposed to night. If you're planning a picnic, you might say “Let's go during the day so we have enough light.”

We divide days into smaller chunks to organize our time: hours, minutes, and seconds. Ancient peoples tracked days by watching the sun's movement across the sky. Today we use clocks and calendars, but we're still measuring the same thing: Earth's steady rotation, which gives us the rhythm of sunrise and sunset, over and over again.