My delicious little 3-year-old cornered me the other day, and was typically adorable: 

P: Hey, Mom?
K: Yes?
P: I need a brother.
K, mildly stressed: Hm. That's a pretty tall order.
P: Because Hettie is a sister and Willa is a sister, but I'm the brother, so I don't HAVE a brother.
K: Brothers take some time, Buddy. And there aren't really any guarantees...
P, resigned: I know. I don't want one right now.
<<thoughtful pause>>
...I want one tomorrow. Tomorrow, I want to go to Costco to see if they have brothers, and then we can buy a real boy. To be my brother. OK?

If someone can think of appropriate, noncommittal response to this, please let me know...