S15E13 Smart & Smarter
After a quarrel with Dr. Hibbert, Homer decides to enlist Maggie for a pre-nursery school. Maggie doesn't get tested because she can't talk. Back home Lisa discovers Maggie can make words and the family goes back to the pre-nursery school. This time Maggie passes the test gloriously and turns out to have an IQ of 167. This is more than Lisa's IQ (159) which makes her unsure about her identity.