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Black holes
A black hole has so much mass tightly packed into a small space that, close up, its gravity is so strong that nothing nearby can escape it.
To simulate a black hole on the gravity well, place a small metal ball on the sheet. Attach a strong magnet to the ball under the sheet and attach a heavy ball to the magnet. The sheet stretches downwards and simulates the infinite stretch of spacetime caused by an actual black hole.
Related resources:
The mystery of black holes – article
Black holes – video
