“Imagine a fly walking on a surface. If the fly walked across a line and disappeared by going around a corner, then that line should be heavy. If the fly walked across a line which marked a change in material in the same plane then it should be light.”
Brian Ramsey, “Trade Secrets”
Or if flies give you the heebie-jeebies, perhaps imagine an ant.
Or a caterpillar, though not a very hungry one like Eric Carle’s.