The World's Greatest Smile! (とびっきりの笑がお！, Tobikkiri no Egao!, lit. Extraordinary Smile!) is one of the events in Tomba!. The next event is similar to this one, but Tomba has to frown to the guard instead of laugh.
On the right side of the Mushroom Forest is a dwarf guarding the road to Phoenix Mountain. Talk to him to activate the event.
Now head over to the forest and leap on the mushroom on the right in the first group of mushrooms encountered to give Tomba the laughing status ailment. Head back to the dwarf and press the attack button to make Tomba laugh. The event is cleared.