What causes day and night on Earth?

The earth's rotations cause day and night. The earth makes one complete rotation roughly every 24 hours. At any point in that rotation, an area of the globe is facing the sun. This is day. As the Earth continues to rotate, eventually this same point will rotate away from the sun. The absence of direct sunlight is what constitutes night.

Clocks....

The presence and absence of sunlight on different sections of the rotating globe.

"Night or nighttime is the period of time when the sun is below the horizon. The opposite of night is day (or "daytime" to distinguish it from "day" as used for a 24-hour period). The start and end times of night vary based on factors such as season, latitude, longitude and timezone."

"The phenomenon of day and night is due to the rotation of a celestial body about its axis, creating the illusion of the sun rising and setting."
Source and further information:
http://en.wikipedia.org/wiki/Night

Giant fireflies

The rotation of the Earth around the Sun causes day and night changes, as well as changes in seasons (i.e. spring, summer, autumn, winter) in some countries.

The Earth takes 24 hours to make 1 complete rotation about its own axis. During each rotation, day and night changes occur (every 12 hours). The side of the Earth that faces the Sun will experience day, while the other side of the Earth will then experience night.

The Earth takes 365 + (1/4) days to make one complete revolution around the sun. The relative position of the Earth to the Sun determines the season in some countries.

the rotating on it's axis

at one point it faces away from the sun which, means it is night but when its facing the sun its day and it takes 24hours to do a full turn.