For the 747-100, one of the configuration items was the software for the navigation computer. In those days (mid-1960s), the concept of software was not widely understood. The weight of the software was 0. The weights people didn't understand this so they sent a guy to the software group to understand this. The software people tried mightily to explain that the software was weightless, and the weights guy eventually went away, dubious.
The weights guy comes back a few days later with a box of punch cards (if you don't know what a punch card is, e-mail me and I will explain). The box weighed about 15 pounds. The weights guy said "This box contains software". The software guys inspected the cards and it was, in fact, a computer program. "See?", the wights guy said, "This box weighs about 15 pounds". "You don't understand", the software guys responded, "The software is in the holes".