>>11289926It's really difficult to explain. I'll give you an example.
Imagine an elevator in space. You feel acceleration, but it's impossible for you to tell if it comes from a gravitating body below you, or from rockets underneath the elevator accelerating you ahead.
Now you shoot a laser from one side to the other horizontally. You see that the laser ends up slightly lower than when there would be no forces present. You explain the effect by the force you feel. But, since you cannot decide whether it's gravity or thrust, those two influences must have the same effect on the laser.
Now you also know that light goes in straight paths only. Of thrust is the reason, the path stays straight, but the elevator moves ahead during the laser flight time, so nothing fancy going on. But if it's gravity and the elevator is stationary wrt the gravitating body, how can the light be bent? Gravity must have changed what it means to have a straight path or, in other words, gravity bends spacetime.
As a corollary, inertial and gravitational mass must be the same, although that is a bit trickier to see. This also leads to the fact that bodies at rest still contain energy (E=mc^2) and some other interesting results.
Something like that. Sorry for wall of text.
