What is gravity? It's one of the easiest scientific concepts to understand the effects of, and one of the hardest concepts to understand by itself. Here — using a large sheet of spandex, some marbles, and a couple weights — is a demonstration that makes the entire concept remarkably graspable.
Commenter EtruscanRaider pointed us to the video in this thread on ways to explain some of the most mind-bendingly complex scientific concepts.
In the video, physics teacher Dan Burns explains how to set up and use the visualization, starting with putting up a single large sheet of spandex (or, as Burns calls it, "space-time"), and then using a combination of weights and marbles to explain everything from gravity's basic mechanics to how gravity lets us plot return trajectories from the moon.
Want to try building a gravity visualizer of your own? There are instructions right here.