He began flipping through the book, scanning the sections where he would expect there to be a significant difference between their world and Earth. Some things you can really only build in one general way, he reasoned. A wagon's got to have its wheels in pairs, for example. You can have a single axle, or two—or even more, if you have a heavy load—but you can't build a one-wheeled wagon and expect it to function. Likewise, the wheel has to be round. Any other shape won't work at all, and probably every civilization will make that discovery fairly early on. A boat has to have a certain type of shape to be useful.