At the end of The Big Bang Theory season 1 episode 13, Penny asks Sheldon and Leonard:
Tweety bird says "I tawt I taw a... ???"
In fact I am French, I saw the episode in French and Penny asks
Titi a dit "J'ai cru voir un... ???"
and Sheldon answers "un Romulien" which vaguely sounds like "un gros minet" (a puddy-tat). So in French it does make sense !
I was wondering what the pun was in English, and I found out that it is also "a Romulan".
But "Romulan" has absolutely no resemblance with "puddy-tat". So what is the joke, in English?