Ambigrams are, as wikipedia puts it, typographical designs that can be read from more than one veiwpoint. Here’s one I did for my blog which reads the same upside down. If you like Dan Brown novels you’ll know already.
The method is pretty easy. Type your word and (in my case) rotate it. Compare them and try to tinker with the original so that it looks more like the rotated image. Of course, you only need to go halfway through the letters if you are doing a simple 180 rotation like mine. For ambigrams that read differently (e.g. come in – go away doormats 😉 ) you’ll need to continue.
If you fancy cheating, try Flipscript.