@willie Marshmallows? (j/k. seriously though: no clue, apart from being a consummate example of the principle. good luck.)
Seriously though, if you have an iOS device, you should give Trainyard a try. Just $2. It’s perfect. l.lbs.im/z (App Store Link)
Don’t tell my wife what time this picture was taken. Also, don’t play Trainyard unless you like awesome games. twitpic.com/9qj6cp