Your precious little daughter comes to you at five in the afternoon and says, "Can I sleep over at Suzy's?" You think about it, and it's not a good idea. It is, after all, a school night. So, no, you tell her.
Then with her big puppy eyes, she says, "Well, can I just go over there to do homework with her?" You think, why not? I don't want to seem like a cold-hearted mom.
Two hours later at seven, you get a ring. It's your daughter in her sweet, sweet voice. "Mom, can I just stay over here for dinner? Suzy's mom already made a plate for me." You think, well... that's nice of Suzy's mom... I can't just say no now.
Then another two hours later at nine, you get another ring. It's your kid again. "Mom, we're trying to finish this project that's due on Wednesday. Can I just finish this? It'll only take another hour or so." Sigh, ok. It's for school, anyway.
Then about two and a half hours later at eleven thirty, you get another ring. Of course, you know who it is. "Mom, we're still not done with the project. And I don't want to do an overnighter tomorrow for this. Mom, you always told me not to leave things to the last minute. And I'm being responsible and doing this project right now. Can I just sleep over here and finish this? I promise we won't play and we'll sleep early. Suzy's mom said it's ok. Please mom, please?" It's almost midnight, and you're getting sleepy yourself, and you did tell her to not leave things to the last minute. Well, ok. Just this time though.
Foot-in-the-door technique well played, kid. Well played.