Connect Four offers a plastic version of the classic tic-tac-toe. This Milton Bradley product first appeared on the game scene in 1974 and has remained popular ever since. Players slip black or red checkers into slots on a vertical board. The object of the game is to have four checkers of the same color in a horizontal, vertical, or diagonal row before your opponent. Because the board itself sits in a vertical plane, gravity becomes a factor. Each player needs to consider gravity in placing his or her four checkers in a row without contributing to the opponent's success. The game's concept wasn't particularly innovative; tic-tac-toe has been around since before pencils, and the checkers used in the game had long been a staple of every nursery and playroom. Still, Connect Four was a hit and remains popular.