Some years ago, while doing research on typography, I made a special font of type that obscured the letters in order to study the effect of spacing. Some of my students happened to look at one of my examples, and decided on their own initiative to look at one of my other examples, and decided on their own initiative to figure out the words that I thought had been obliterated. To my great surprise, they were not only able to reconstruct a highly complicated and abstruse text, but enjoyed every minute of the task! They were so hooked on the problem they spent the whole night puzzling it out, and the next morning they challenged their friends to do it.
Knuth, Donald E.
Word Ways: Vol. 20
, Article 13.
Available at: http://digitalcommons.butler.edu/wordways/vol20/iss3/13