They are finding out how the universe works. This is spontaneous research which is inherently gratifying, often joyously gratifying. How wonderful to find out to use gravity as an accelerator or a brake. Nobody is around to tell you or to give you the name gravity, but you learn quickly that the greater the drop, the more it hurts your legs. That is what Galileo's work with falling bodies was all about.
Children learn about tension.
They have got to tear a great many things apart to find something that won't tear, that they can spontaneously grab for to arrest the falling and anticipate leg shock or break. They don't have to know the names tension, compression, gravity, or acceleration, but they have to get very familiar with such phenomena before a sound emanating from somebody's mouth can develop a word meaning experience. Playgrounds provide children with experience-fortified gratification of physical research.