The Einstein Collection E=Mc² Metal Puzzles - 3D Brain Puzzles by Professor Puzzle. Contains 3 interlocking metal wire puzzles. Medium difficulty. Develop your problem solving skills - Can you separate the metal puzzles to form the famous equation? A perfect gift for any Physicist or Mind Puzzle enthusiast. Part of Professor Puzzles range of Wooden 3D Brain Teaser Puzzles & Adults Brain Teasers., Manufacturer: Professor Puzzle