Here's why:
* Every object with mass exerts a gravitational force on every other object with mass. It doesn't matter how big or small the objects are, they all attract each other.
* The force of gravity is proportional to the product of their masses. This means that the more massive the objects, the stronger the gravitational force between them.
* The force of gravity is inversely proportional to the square of the distance between their centers. This means that the farther apart the objects are, the weaker the gravitational force between them.
Because these principles apply to all objects with mass, regardless of their size, shape, or composition, the law of gravitation is considered universal.
