Fling a ball hard enough, and it never returns. You don't see that happen in real life because the ball must travel at least 11.3 kilometers (7 miles) per second to escape Earth's gravitational pull. Every object, whether it's a lightweight feather or a gargantuan star, exerts a force that attracts everything around it. Gravity keeps you anchored to this planet, the moon orbiting Earth, the Earth circling the sun, the sun revolving around the galaxy's center and massive galactic clusters hurtling through the universe as one.

Gravity and three other fundamental forces hold the universe together. The strong nuclear force keeps particles in an atom's nucleus from flying apart. The weak nuclear force causes radiation in some nuclei, and the electromagnetic force performs critical tasks such as holding a molecule's atoms together. Although the sun's gravity grips planets billions of miles away, gravity is the weakest fundamental force.

Mass, sometimes confused with weight, is the amount of matter an object contains -- as mass increases, so does gravitational pull. Black holes, astronomical objects often seen in science fiction movies, are so massive that light can't escape them. A grain of salt's gravity is much smaller because it has less mass. Weight refers to the force an object's gravitational pull exerts on other objects. Weight can fluctuate, as witnessed on lunar missions where astronauts weighed six times less than they do on their more massive home planet, Earth.

Books and articles may talk about space station astronauts floating in "zero gravity." Earth's gravity still exists in space and is actually only 10 percent weaker up where the space station orbits. Astronauts float because they're falling toward the planet and circling it so quickly that they never reach the surface. Even though an object's gravitational pull weakens with distance, it extends outward to infinity. In other words, Earth still attracts bodies at the edge of the universe.

In 1687, Issac Newton informed the world that "gravity really does exist." Before then, nobody knew that. Today, Newton's theories explain how heavenly bodies move and help people predict the way gravity influences life on Earth. Projectiles, for instance, follow paths as predicted by Newtonian calculations. Centuries later, Einstein theorized that objects warp space, resulting in gravitational pull. Visualize this by placing a bowling ball on a mattress to cause a depression. If you put a marble on the bed, it rolls toward the depression. In Einstein's theory, the massive sun would be the bowling ball and Earth would be the marble that moves toward the sun along with all planets, asteroids and comets.

If the sun suddenly lost 95 percent of its mass, the Earth would not feel the effect instantaneously, says Einstein. He predicted gravity waves -- ripples that travel through space causing it to stretch and squeeze. Rapidly orbiting binary stars and massive black holes merging are some astronomical objects that cause gravitational waves. These waves are too tiny to measure coming from small objects, so scientists attempt to detect them using a special observatory. Proving the existence of gravitational waves will mark a milestone in the quest to understand gravity.