The universe is like a piece of cloth, planets deform the cloth, forming a curvature. Seeing a metaphor, use GPT to study and learn: You can first imagine space-time as a taut rubber blanket (2D). Put the sun up → the cloth sinks a piece. The Earth is like a small ball → attracted by the "depression", circling along the depression. The Earth is not 'pulled', but 'rolled along a curved surface'. The meaning of this metaphor is that celestial bodies bend spacetime, and gravity is actually the object taking the straightest path in the curved spacetime. But this metaphor is also inaccurate, because the universe is not a membrane, not a cloth, and not a two-dimensional object. The metaphor of "cloth" is used because the human brain cannot imagine the curvature of four dimensions (3 spaces+1 time), so it can only be reduced to 2D. The true meaning is that mass and energy change the definition of "distance" and "time" itself. This metaphor is more profound than any other. The sun does not "pull" the earth, but rather causes changes in the nearby spacetime structure. The earth naturally takes the "shortest path" (geodesic) within this structure, just like you push a cup and it naturally slides along the tabletop, not the table pulling it. Quality changes the measurement of spacetime, while curved spacetime determines the path of objects. Do not understand gravity as' pulling force ', but as: a straight line is no longer straight, the shortest path bends, time is distorted, space is altered, and the causal structure is modified. You see the Earth orbiting around the Sun, which is actually the most effortless path to take in the curved space-time. The phrase 'the universe is like a piece of cloth' is an inaccurate metaphor, but its true meaning is that mass and energy can alter the geometry of spacetime, leading to the gravitational phenomena we experience. This sentence is the most consistent explanation with Einstein's theory. General relativity tells us that space is not a background, but a participant, not a ball running on a table, but the table itself bending and changing. Time is not uniform, and mass slows down time (gravitational time dilation). Motion is not forced, objects naturally move along a "straight line", but the definition of a "straight line" is changed by spacetime itself.