Look at the last line of the chapter. What is the old saying that Scout refers to here? How is this an appropriate idiom for this point in the trial?

The last sentence in chapter 17 is:

His back was to us, but I could see his broad shoulders and bull-thick neck. He could easily have done it. I thought Jem was counting his chickens.

Jem feels Atticus's case is solid and Tom will be set free. He is taking for granted that Atticus will win the case. Jem is effectively, "counting his chickens before they hatch." Scout has some doubt because everybody can see how big and powerful Tom is.

What does Atticus do that makes Mayellab Ewell think that he is making fun of her