Select a puzzle and then press number keys to drop pieces. When you form a row of balls with matching colors, the row disappears. If you clear the board, you win. If you form a mismatching row, you lose. Press backspace to undo.
Some puzzles have no solution. If you think a puzzle has no solution, click the "No solution" button. If you're right, you win; if you're wrong, you lose. This action cannot be undone.
Lorentz, Richard J., 2001. Creating difficult instances of the Post Correspondence Problem. In Computers and Games: Second International Conference, pages 214–228.
Zhao, Ling, 2003. Tackling Post’s Correspondence Problem. In Computers and Games: Third International Conference, pages 326–344.