In The Color Purple, what did the mayor's wife ask Sophia, sparking the confrontation?

In The Color Purple, the mayor's wife
Miss Millie asks Sophia if she wants to be her maid, and this question sparks a
confrontation that lands Sophia in jail.  Sophia has brought Buster and the children out
grocery shopping, and in the store, she runs into Miss Millie.  Miss Millie thinks that
Sophia's children are cute, and she compliments them.  Then she asks Sophia if she wants
to work as her maid.  Sophia is insulted because she does not want to be the maid for
any white woman, so she refuses Miss Millie's request.  But the mayor thinks that Sophia
has been rude to his wife, so he slaps Sophia in front of everyone in the store.  Sophia
is known for not taking abuse from anyone, so she defends herself against the mayor. 
The police arrive and draw their guns, and Sophia is then beaten by them and dragged to
jail.  Ironically, Sophia ends up working as Miss Millie's maid to get out of the
jailhouse and serve her sentence.