Here's why:

* Gravity is a force: Gravity is a fundamental force of nature that attracts objects with mass towards each other.

* Joules measure energy: A joule (J) is the unit of energy, which is the ability to do work.

* Work is force applied over a distance: Work is calculated as force multiplied by distance.

Therefore, gravity itself doesn't contain energy. However, the potential energy of an object due to its position in a gravitational field is measured in joules.

Example:

If you lift a 1 kg object 1 meter off the ground, you've done work against gravity and given the object potential energy. The amount of potential energy gained is approximately 9.8 joules (1 kg * 9.8 m/s² * 1 m).