Physics Puzzles

I love puzzles - especially those that require you to look at problems from a different perspective or require a non-conventional approach.
The two puzzles below are ones I often pose to my best A level students (often as preparation for university interviews). The first one has (for an A level Physicist) an immediately apparent solution but, upon thinking about it closer, the solution doesn't quite work. when I pose this to students I don't necessarily want them to arrive at the correct answer (which you can probably find if you do a search for "3D glasses" ) I am more interested in the way they think about the problem.
The second problem requires you to think very differently about uncertainty - if you follow a traditional approach you will almost certainly not solve the problem. The puzzle is actually based on a problem I came across whilst on a Maths course.
3D Glasses Mystery
Centre of Mass Puzzle