Here is a piece of paper:
Fold it once, and you can get a shape with 9 corners:
Starting with a rectangular sheet of paper and folding twice (along any line), what is the largest possible number of corners that may result?
Rule for counting corners: both concave and convex corners count (as you can see in the example), but they must not be "covered" by the rest of the paper. In other words, you are counting the corners of the silhouette.
Credit: Alex C Weiner