CompuServe Messages

I have a friend who talks and lunches with Don Knuth now and then. He is working on the next volume, and in it, he includes several computing problems he's dreamed up that should tax everyone's brain… my friend tried to explain one, I think it went like this: take the set of all the five-letter words in English. You can permute from one word to another by changing one letter at a time. Find the shortest path between the words "blood" "sweat" and "tears." (I think I've forgotten some twist on the way you permute from one word to another…)