Laughter replaces screams as the energy source at the end of Monsters, Inc. The change begins when Sulley and Mike discover that Boo's laughter produces far more power than the screams collected by the factory's traditional scare operation.
For most of the film, Monstropolis depends on frightening human children. Scarers enter children's bedrooms through linked closet doors, and their screams are captured in canisters to power the city. The system is increasingly strained because children are becoming harder to scare, creating an energy crisis for Monsters, Inc.
After Waternoose is exposed and arrested, Sulley retools the factory around laughter instead of fear. The film explains that laughter is ten times more powerful than screams, turning comedy into the new industrial energy system. Mike becomes a leading jokester while Sulley helps run the company. This is why the ending is more than a change in workplace policy: it resolves the city's energy shortage and reverses the monsters' mistaken belief that human children are dangerous.