The question is a rather awkward one, so it's understandable that Cal shifted the responsibliity of answering it onto Atticus. Scout is talking about the Robinson case, and she asks, "What's rape, Cal?" Cal quickly tells Scout that "It's somethin' you'll have to ask Mr. Finch about...he can explain it better than I can." Then, she quickly changes subject, asking if they were all hungry, and commenting on the sermon that the preacher gave that morning. Scout does in fact ask her father later on, and Atticus gives a straight-forward explanation, letting her know that it is "carnal knowledge of a female by force and without consent." Scout, being so young, probably doesn't grasp the full meaning of this, considering her response of "Well if that's all it is why did Calpurnia dry me up when I asked her what it was?" But she stops pestering him about the issue, and they move on to other subjects.
In Chapter 12, Scout asks Calpurnia, "What's rape, Cal?" (124). The question is in reference to Tom Robinson being accused of raping Mayella Ewell. Calpurnia's exact response to Scout's question is, "It's somethin' you'll have to ask Mr. Finch about. . . . He can explain it better than I can" (124). She then immediately changes the subject by asking if the children are hungry.
The reason why Calpurnia responds this way is another matter. Calpurnia, being the wonderful cook, caretaker, and mother-figure that she is, knows that this is a conversation that Scout should have with Atticus. A violent crime like rape should not be discussed (and certainly not described) to any child by anyone other than her parents. The intelligent Calpurnia also knows that something forbidden suddenly seems very interesting to curious children. However, a growing child's stomach usually takes precedence over absolutely everything else.
Scout asks " whats rape?"