Say you have a box with a round marble on it. Classically I would have no problem describing that, but how is it done in general relativity? Do one just assume that the marble is round and that the box is ... boxy... in the 3D subspace at a certain foil of time? (Foliation is the correct term right?) Or does one iteratively perturb/deform the (materials) geometry, until Einsteins laws with the stress energy tensor etc is satisfied? I'm asking because I have a gut feeling that a round marble isn't round in general relativity. And also because every single textbook I have read have been void of examples. Thanks!