So that Charlotte can conduct important business in peace, and so Drew & Stu can watch tennis, they tell Angelica that they'll buy a Cynthia Dream House and a mini sports car for her if she stays quiet. She ends up taking this ""quiet"" thing very seriously by not saying a peep, not even when the Rugrats pay a visit. Eventually, the Rugrats' racket ended up making her more angrier, with no way for her to vent her anger; also, her silence ended up affecting Charlotte's business deal, rather than helping it.
