Gravity is one of the most mysterious forces. Gravity is often problematically explained in an analogy wherein it is described as a property of time-space. This explanation was first offered by Einstein and regards time and space as a 3-dimensional, stretchable fabric. Check out this demonstration high school physics teacher at Los Gatos High School in California.
It’s easy to see the effects of gravity but the closer you examine the more vague the cause of the phenomenon seems to become. Physicists have been able to agree for hundreds of years: the more mass an object has the more it attracts other objects. A grade school understanding of gravity often leaves children thinking gravity is simply the force pulling everything downward, toward the Earth. Really, every object that has mass is being pulled towards each other, and the largest mass nearby anything you can see is the Earth itself.
So, you can use a stretchable fabric sheet to demonstrate but because the exact cause is not fully understood, scientists have long struggled to concisely- or even accurately – explain gravity. In short: everyone can see the effects but no one has proven the cause behind the effects.
Perhaps the best short clip on the subject is from the original cosmos series, with Karl Sagan. Check it out~!