Morticia Joins the Ladies League - S1-E6
Continuity mistake: As Pugsley comes out of the closet counting his money, he's dropping coins from his right hand to his left. A second later, it's reversed, and his facial expression changes in the same moment.
Answer: Basically from what I understand, they stuck the innards of a small flashlight into the base of the bulb and put a little switch on it. When he put it in his mouth and bit down, his teeth would hit the switch, which would turn on the flashlight and made the bulb light up.
TedStixon