In an interlocking puzzle, one or more pieces hold the rest together, or the pieces are mutually self-sustaining. The aim is to completely disassemble and then reassemble the puzzle. Burr #306 is a classic example of an interlocking puzzle. Both assembly and disassembly can be difficult – contrary to assembly puzzles, these puzzles usually do not just fall apart easily. The level of difficulty is usually assessed in terms of the number of moves required to remove the first piece from the initial puzzle.