There's nothing more contagious than the laughter of young children; it doesn't even have to matter what they're laughing about.
Among other possibilities, money was invented to make it possible for a foolish man to control wise men; a weak man, strong men; a child, old men; an ignorant man, knowledgeable men; and for a dwarf to control giants.
Most adults are knowledgeable to a child, but ignorant for their age.