Caption: Einstein's general relativity (GR) shows that mass-energy causes spacetime to curve---in way that is NOT easy to perceive for relatively small mass objects like the Earth---but that can be measured.
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It's easy to understand curved 2-dimensional spaces like the surface of a sphere. The curvature is really in 3-dimensions which very hard for humans to visualize though mathematically it can be treated.
One has to find a self-consistent GR solutions for physical systems dominated by gravity. This is NOT easy and, except for a few simple cases, requires masssive numerical solutions on the computer.