The One With Frank Jr. - S3-E5
Stupidity: The friends discover that Joey's built a ridiculously oversized entertainment center only after they've pushed it upright. Cute concept, but there's no way that's when it would have become obvious. First of all, all they had to do was see it stretched across the doorways when it was lying flat on the floor. Second of all, Chandler had been watching it get built; even if he wasn't really paying attention, it would have been very immediately obvious at a single glance. And Joey's a space cadet, but there's no way he wouldn't have noticed either.
Answer: Usually by the seventh night of Hanukkah you don't know what to get the other person, and you will probably resort to getting the other person something insignificant. Chandler is basically saying that the pen is a stupid gift.