Einstein’s path to the discovery that mass warps spacetime began with a profound thought experiment in 1907, which he later called, the happiest thought of my life.
It started with the question he asked to himself,
"If a person falls freely from the roof of a house, will they feel their own weight?"
When Einstein realized that a falling person would feel weightless. In their perspective, gravity would effectively disappear.
This led to several game-changing realizations;
First one is,
The Equivalence Principle;
He concluded that gravity and acceleration are essentially the same thing. If you were in a windowless elevator in deep space being pulled upward at a constant speed, you wouldn't be able to tell the difference between that and standing still on Earth.
Next one is,
Gravity as Geometry:
Because gravity affects everything exactly the same way regardless of mass, Einstein reasoned it isn't a "pulling force" like magnetism. Instead, it must be a property of the environment itself, which we know now as the fabric of spacetime.
Last but not least,
Warping Spacetime;
If gravity is equivalent to acceleration, and acceleration can curve the path of light, then gravity must also curve light. The only way for light which always takes the shortest path to curve is if the ground it's traveling on spacetime which is itself curved by mass.
For those just looking for fancy science fact it's just one of the discoveries about nature but Einstein himself it was the way of living life.