Sarah Tuttle is a girl in "The One Where Rachel Quits" who was part of the Brown Birds selling cookies to people. She befriends Ross after he breaks her leg when he accidentally hits her with a tennis racket bag (with a racket inside) while she was selling cookies, and Ross sells the cookies on her behalf.
Sarah was a little girl who enjoyed to spend most of her time daydreaming about outer space. She had a very irresponsible father, who used to bet on slot machines the money that Sarah's grandmother left her for her college. He also made her clean apartments in order to be able to afford a trip to the Taj Mahal (the casino).
Sarah wanted to sell cookies for the Brown Birds because the girl who sells the most would win a trip to Spacecamp, and gets to sit in a real space shuttle; but this goal was put on risk after Ross accidentally broke her leg and left her temporarily incapacitated. She didn't want to win the prize for the second place (a ten speed bike) because she rather have something her father couldn’t sell. She also had to watch the news about the astronauts on the TV of the neighbor through the window because she didn't have TV (hinting that her father had also sold it).
Feeling regret for ruining her dreams, Ross took her place and started to sell the cookies on her behalf. Sarah told him that the girl who won last year sold 475 boxes; and after much effort, Ross manages to sell 517. However, he loses to a girl who sold 871 and to another one who sold more than 2000 (after she loaned her uniform to her 19 year old sister, who went to sell the cookies to the U.S.S. Nimitz); thus not winning any prize.
At the end of the episode, Ross takes Sarah to Chandler and Joey's apartment where they have made a private space camp for her. Ross puts her on a seat decorated with foil and he, Joey and Chandler all turn her around on the chair while pretending to be aliens and throwing an asteroid. The camera turned to her, revealing a big smile on her face, obviously showing she was enjoying it.