I was drawing with some children when a 4 year old boy came to me with a problem: His crayon wouldn't work.  That caught me by surprise, so I asked him why it wouldn't work.  He then proceeded to scribble on a sheet of white paper with his white crayon and said, "look, it's broken."  We then spent the next few minutes gathering all the white crayons so we could test them to see which ones work.  It turns out they were all "broken".