This may seem like a really basic question (I'm teaching lower school science, and it's not my specialism!), but: What causes the force which balances gravity, which I think is called "ground reaction force?" All the descriptions I've seen say that the ground "pushes" up on a stationary object, preventing it from falling through the ground. However, this description seems very "active." Are there some kind of elves heaving away underground to keep the earth from caving in? A fair amount of searching has not provided any more convincing description. Could someone here please try to enlighten me? Many thanks in advance, Robin.