In "To Kill a Mockingbird" what have Jem & Scout learned about Atticus's past that surprised them?
Before chapter ten, Jem and Scout thought that Atticus was a bit of a fuddy-dud dad, one who just sat around reading books all day, was kind-of old, boring and lame. In fact, the chapter opens with Scout's frank opinion that "Atticus was feeble," and that when other kids bragged about their fathers, Scout felt that
"there was nothing Jem or I could say about [Atticus]...our father didn't do anything...that could possibly arouse the admiration of anyone."
So, even though Atticus had his strong points, and taught the children about equality and human decency, he wasn't super cool or strong, or anything spectacular that they could brag about at school.
In chapter 10, a rabid dog starts heading down their street, and much to Jem and Scout's surprise, Atticus takes Heck Tate's rifle and very efficiently, with one shot, kills the dog from a good distance away. Jem and Scout are stunned--was that their dad that just wielded a rifle with such ease and expertise? Now THAT'S cool. They had no idea that he was such a good shot; they didn't know because Atticus never told anyone. According to Miss Maudie, he didn't want to brag, or to make others feel bad because he was so good at something. So, Atticus's secret talent of excellent marksmanship with a gun was a complete surprise to his two children. They come away from the experience with quite a bit of pride and admiration for their previously "feeble" father. I hope that helped; good luck!