/**
         * Called when a view has been clicked.
         *
         * @param v The view that was clicked.
         */
        @Override
        public void onClick(View v) {
          // if it is the first time a user clicks on a tile,
          // this should start the game
          if (!mGameInProgress) {
            startGame();
            return;
          }

          // any other clicks should get checked to see if answer was right
          switch (v.getId()) {
            case R.id.button_game_0:
              break;
            case R.id.button_game_1:
              break;
            case R.id.button_game_2:
              break;
            case R.id.button_game_3:
              break;
            case R.id.button_game_4:
              break;
            case R.id.button_game_5:
              break;
            case R.id.button_game_6:
              break;
            case R.id.button_game_7:
              break;
            case R.id.button_game_8:
              break;
          }
        }