In the beginning of the trial, Atticus and Mr. Gilmer were talking to Heck Tate and questioning him what he saw, why he went down to the Ewell's and didn't call a doctor on Mayella's injuries.
Mayella is now up to tell her side of the story, and she is nervous and has no idea what to say.
Her story was that she went inside to get something and Tom came in and attacked her, but they figured out that all her injuries were on the right side, so someone left handed had to have hurten her.
When Mayella was talking, Tom stood up and told his story, when the judge asked him what happened to his hand.
He explained that his left hand got stuck in a cotton gin, so therefore it couldn't have been him if his left hand was messed up. He told the judge that Mayella was the one who came upon him and attacked him.
I got it stuck in a cotton gin when I was little.
After Tom and Bob being there, they were told to write their names on an envelope to see which one was left handed and beat Mayella.