It only appears briefly in a few episodes in the earlier series (1-5), although it may appear in later episodes (I haven't seen all of them). The episodes I remember seeing it in are: 2.15 "Three Days of the Kahndo" (they go on holiday to Mexico), 5.03 "I Don't Wait to Wait for Our Lives to Be Over" (Joseph enters puberty and has a big growth spurt) and 5.18 "The Trouble with Gribbles" (Nancy worries that she's aging so Dale sues his tabacco brand).