Select a piece to drop it. When you form a matching row of balls, the row disappears. If you clear the board, you win. If you form a mismatching row, you lose.
Some puzzles have no solution. If you think a puzzle has no solution, click the "Give up" button. If you're right, you win; if you're wrong, you lose.
Poco is based on Post's Correspondence Problem. This problem is in general undecidable: you can write a program to solve a puzzle if it has a solution, but if a puzzle has no solution, the program might run forever. There are puzzles for which no one knows (yet) whether there is a solution; the simplest of these is Level 20.
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.