[math-fun] shapes with constant surface

Actually, a nice 3D solution occurs to me... which unfortunately seems useless and is kind of a cheat. Make a torus, i.e. the points within distance <=1 from a large circle. Also, make that circle itself be a "surface" (of area=0, or you could make it be another torus so the area was positive). The torus erodes from both the outside-in and the inside-out and always has constant surface until the instant it vanishes. Cute... except not useful for fertilizer purposes since only the surfaces ACCESSIBLE from the outside are allowed to erode in real life. (As I said originally, I believe I do have a genuine 3D solution, but it is messy.)