Grab a friend, pick a robot (Atlas or P-Body), and start testing. Co-op in Portal brings a ton of new angles to the familiar puzzle game. Each player has their own portal gun, with their own portal colors. Teaming up to solve complex puzzles is very rewarding, and there are plenty of co-op missions both in the campaign and the Steam workshop. Add in the joy that comes from dropping your buddy to his death, and you can't go wrong with Portal 2. It's like a team building retreat!