Uncertain Principles: On the Quantum Physics of Whiteboards

The black markers write very clearly on the board, but when I attempted to erase the board at the end of class, the erasers just sort of smeared the ink around leaving a greyish smudge on the board.

From this, we can deduce that the operator W, which describes wiping the board clean, and the operator M, which describes making marks on the board, are non-commuting operators.