Impossible Flap?

Try to make this simple model and you will see that it is surprisingly difficult to do. There is no trick. This is made from one piece of paper cut and folded just right. Even when you know how to make it, it still may take you a while to get your head around exactly what is going on. Challenge your friends to reproduce your model. Let them look at it, but not touch it.

Steps

1. Since this is a puzzle, I won't post steps here. However, if you must know the secret, you can download the pattern and cut and fold where marked. Cut along the solid lines, fold valley folds along the dashed lines and mountain folds along dot-dashed lines.