Math Designed to Make Kids Think

I have posted several math problems designed to invoke thinking, on the refrigerator with out saying a word. The kids are enjoying it thoroughly.

Each sheet remains up for 3-6 days while everyone has a chance to ponder, explore and write down their thoughts and solutions.

Here's what's being posted.
Algebra on Rectangles from Let's Play Math and Math Pickle
Visual Patterns from Let's Play Math and VisualPatterns.org
Math Without Words from James Tanton (this website also contains many excellent videos)
Pondering Large Numbers from Let's Play Math
How many ways can you make 12? - from a homeschooling mom on a Math message board

Those are the ideas I have so far. If you have any suggestions I would love a comment.
For more educational ideas check out these blog hops.