I seem to recall reading an analogy describing the total mass/energy of a jack-in-the-box. This said that a compressed spring insided a box would "weigh" more than the same when relaxed (albeit immeasurable small). This makes sense given the equivalence of energy and mass. But what if the spring is stretched out (and held out inside the box). Does the box now "weigh" more again due to the energy stored in the streched spring or is the sign preserved making the whole deal "lighter". I'm sure this analogy was given by a respectable physicist when describing negative energy due to gravitational potential - I just can't recall where.