Here's why:

* Newton's Law of Universal Gravitation: This law states that every particle in the Universe attracts every other particle with a force that is:

* Proportional to the product of their masses. The more massive the objects, the stronger the attraction.

* Inversely proportional to the square of the distance between their centers. The farther apart the objects, the weaker the attraction.

So, gravity is indeed a pull between two masses. It's a fundamental force that governs the movement of planets, stars, galaxies, and even the apple falling from a tree!