A Cartesian diver, on the other hand, has an internal space that, unlike a hydrometer, is not rigid, and thus can change its displacement as increasing external air pressure compresses the air in the diver. If the diver, instead of being placed in the classic plastic bottle, were floated in a flask on an elevator, the diver would respond to a change in air pressure. Similarly, a non-rigid container like a toy balloon will be affected, as will the rib cage of a human SCUBA diver, and such systems will vary in buoyancy. A glass hydrometer is rigid under normal pressure, for all practical purposes.