Fun Mathbox for math lovers

For fun and difficult math!

I kept making a certain typo in the word "matchbox" until I decided it must be a sign. So, here it is - a MATHbox swap.

What do you need to do for this swap? It all grows logically from the swap name:

• take a box
• decorate it and fill with fun math

Now, to be a bit more detailed...

• I want each of us, for this swap, to save a small box from trash. So, unless you are a chemistry teacher and have huge amounts of empty matchboxes, don't use one of them, and also don't make one yourself. You can take a box that used to contain pills, condoms, staples, embroidery thread, mints, cheese, chocolate, cookies... - anything, as long as it a small clean cardboard box that you'd throw away if it wasn't for this swap.
  Let the box be no smaller than a 32ct matchbox, but small enough to fit into a 10x15cm (4x6in) bubble envelope - except if you want to use a box that has some other shape than the usual cuboid (or rectangular) box shape - but still a mathematical one, of course. Decorate it on some mathematical theme (so nobody will know what you had in there). Don't forget that there's more to math than numbers...
• What is fun math? Any kind of math-related jokes; funny, fake or just interesting math problems; math puzzles - logic problems, sudokus of all kinds, KenKen, Slitherlink, nonograms, light up, Hashiwokakero (bridges), Shikaku (rectangles), Nurikabe (Islands in the Stream), Dominosa, mazes, Kakuro, fill-in-crosswords, etc etc etc. You can cut problems out of magazines or books, copy from them (giving credit where due), or even create the jokes/problems/puzzles yourself. Don't stuff your box with pages from only one magazine, though (this is why I suggested copying, making them smaller - to fit more) - let there be at least four kinds of fun math in the box. You are also welcome to present your fun math in an even more fun way - as an ATC or inchie, for example.
  In this swap, you are allowed to send as difficult puzzles and problems as you wish, but please don't get too specific (like my master thesis theme that about five people in the world have ever worked with).

This swap is open to swappers with at least a 4.9 rating, no unexplained 1s and 3s, and at least three ratings for snail-mail swaps. I also reserve the right to ban as I deem necessary.