This one's in the "Omelette du Fromage" episode (one of my favorites...hehe).

At the beginning of the episode, Computer gives Dexter his "To Do" list. It includes feeding the fish, recording the hamster's exercise, studying for his French test (of course), and others. Dexter does them all, except the French test. The computer makes a note of this and Dexter quips that he has more important things to do. As he finishes his important game of Tic-Tac-Toe, we see Computer in the background, and the last 3 items on the list are totally different: Split the Quark, Name the Galaxy, and something else.

Wha?