* Gravity is a force of attraction. Every object in the universe with mass exerts a gravitational pull on every other object. The more massive an object, the stronger its gravitational pull. The Sun, being the most massive object in our solar system, has a very strong gravitational pull.
* The planets are constantly falling towards the Sun. This may seem counterintuitive, but imagine throwing a ball horizontally. The ball will eventually fall to the ground because of gravity. The planets are doing the same thing, but they're moving so fast that they constantly miss the Sun as they fall.
* Orbital velocity. The planets are moving at just the right speed to maintain a stable orbit around the Sun. This speed, called orbital velocity, is a balance between the Sun's gravity pulling the planet inward and the planet's inertia (its tendency to keep moving in a straight line) pulling it outward.
Imagine a ball on a string:
* The ball represents a planet.
* The string represents the gravitational force of the Sun.
* You are swinging the ball around.
The faster you swing the ball, the less likely it is to fall towards your hand (the Sun). The speed at which you swing the ball determines its orbit.
Therefore, the planets move around the Sun because the Sun's gravity pulls them inwards, and their orbital velocity keeps them from falling into the Sun.
