You play red and the computer plays yellow. Tap a column to drop a disc. It falls to the lowest empty spot. Be first to get four of your discs in a row: across, up and down, or on a slant. Pick Easy, Medium or Hard. Each game gives you 2 hints and 3 undos. On a keyboard, use the left and right arrows to aim, Enter to drop, or keys 1 to 7.
Try a red drop without changing the game. Check whether yellow can win on its next move. This checks one reply, not a full strategy.