Here's why:

* Repetitive motion: The pendulum continuously repeats its motion of swinging from one side to the other and back again.

* Regular interval: The time it takes for the pendulum to complete one full swing (its period) remains constant, assuming the conditions remain the same.

Other examples of periodic motion include:

* A vibrating guitar string: The string oscillates back and forth, creating a sound wave with a specific frequency.

* A planet orbiting a star: The planet's path around the star is repetitive and takes a predictable amount of time.

* A bouncing ball: The ball repeatedly rises and falls, with each bounce taking approximately the same amount of time.

* A spinning wheel: The wheel rotates continuously at a constant speed, making a full rotation in a specific period of time.

These are just a few examples of the many types of periodic motion found in the natural world and in engineered systems.