Dexter's class runs an intelligence test. His result: average. The boy genius is shocked to see this, and then decides to leave his laboratory behind, and just live as an average joe - even if the only clothes he has, other than his regular laboratory outfits, were ones he hadn't wore since he was a baby! He then befriends an average kid, Jimmy Smith. In the end, the Pentagon calls to reveal that the computers made a mistake in evaluating Dexter's test, and that his true result is "genius".
The baby clothes Dexter wore re-appeared in the season 3 episode "If Memory Serves".