Here's a quick explanation of general relativity -- specifically, how gravity is actually the curvature of the space-time continuum.

We commonly think of space in three dimensions -- length, width, and height. I add to that a fourth dimension, time. You might visualize space as a grid, with the three dimensions labeled x, y, and z.

It's not easy to visualize four dimensions. One way to do it is by taking away one of the three spatial dimensions, the vertical z axis, for example, and replacing it with time.

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