Two players take turns dropping discs into a board of seven columns and six rows. A disc falls to the lowest free space of its column, so the only thing you choose is the column. The first player to line up four of their own discs wins, vertically, horizontally or diagonally. If the board fills up with no line, the game is a draw.
The rules take a minute to learn and the game stays tricky. The middle column is the most valuable, because more lines of four pass through it than through any other. Before each move, check whether your opponent has three discs in a line with the fourth space free. The surest win comes from two threats at once, because your opponent can block only one. Since every move frees the space directly above your disc for your opponent, never play right below a space that would win the game for them. And if a higher space would win it for you, leave the space below it empty until your opponent is forced to play there.