At the end of the nineteenth century, Sam Loyd, pre-eminent American puzzle columnist and a trained engineer, presented the Mars Canals puzzle to his readers around the world: "Here is a map of the newly discovered waterways in our nearest neighbor planet, Mars," he wrote. "See if you can make a tour of all the towns and back to the point of beginning without going through any one spot twice. Commence at the south pole from the letter T and spell a complete sentence, using each letter once." It is said that he got 50,000 responses in the mail, all saying there is no possible way. But there is. Click on the canal to open the sluice gates. Click in the upper corner to close them all again.