>Slightly off topic : there was an article in one of the Sundays today about someone who had worked out the weight of an eBook (using e=mc2). Apparently every book you add to your Kindle increases its weight by .000000000000000001 gms (or the equivalent of one E-coli bacteria) :-}

Back in 1991, we have empirically found out that a fully charged UPS is heavier than an empty one.

More due to the fact that when we got them, the truck was right at our window and we only had to pass them 2-3 meters to the shelf. When we sold them, we shipped them by cars, and these were parked 50m away from the room. Still, doesn't invalidate the conclusion.