What does it tell you about Amanda when she responds, "I'm incubating an egg," to Maniac's question about what she's doing up so late?

The quote in question comes in chapter 18 of Maniac Magee. This is the chapter that sees Maniac set on leaving Amanda's house because of what was written on the outside of her house in the previous chapter. Amanda tries everything to convince Maniac to stay. She pleads. She reasons. She even threatens at one point; however, Maniac isn't budging. He takes off and doesn't come back by lunch or dinner. Amanda is worried for him, so she goes out looking for him. She returns without having found him. Amanda is stubborn and worried, so she decides to set herself up on the couch and wait for Maniac to return. Maniac does eventually return, and asks why Amanda is still awake. Her response is the egg quote from the question.

"What're you doing up so late?" he said.
"I'm incubating an egg," she snarled.

The quote tells readers all kinds of things about Amanda. First, it tells us she's angry. That's why the comment comes out a snarl. Second, the comment reminds readers that Amanda is smart. She always has her nose buried in a book, and she's constantly learning. The comment tells readers that she is scientifically knowledgeable about the importance of egg temperatures. Third, the comment shows that Amanda has a quick wit and hasn't lost her sense of humor. She's been worried sick about Maniac all day, yet she is still capable of cracking a joke instead of crying or ripping Maniac's head off.