Grace Hopper was a famous American Navy officer and computer scientist who passed away in 1992. This vintage footage of her explaining nanoseconds in physical terms just went viral this weekend, and is featured on GeeksAreSexy and VideoSift

Hopper explains to a class room how a nanosecond is a billionth of a second, a number to small to really grasp. So she expressed a nanosecond with a string of wire, just under a foot to be exact. That’s how far electricity can travel in a nanosecond. To compare, she also shows what a microsecond looks like physically–a wire nine hundred and eighty-four feet long.