Cooperation / Divided Attention
To get to the exit, Fireboy and Watergirl must help each other by pulling levers or helping one another over hurdles. The levels are not won until both are at their exit doors.
Played as a single player, the game requires you to divide your attention between them, making sure neither gets into trouble while you focus on the other one. Played with two players, it's great training in cooperating with others.