What is the first thing that Scout finds in the oak tree on the edge of Boo Radley's property? How does Jem react when he learns about Scout's discovery?

As a first grader, Scout gets out of school thirty minutes earlier than Jem does, which means she must walk home without him. She runs past the Radley place each day because the children have made Arthur Radley into a legendary monster, even though they've never seen him. One day as she is running past, she notices a piece of tinfoil "winking at me in the afternoon sun" from one of the two oak trees on the Radley lot. The tinfoil is sticking out of a knot-hole that is about as high off the ground as Scout is tall. She reaches into the knot-hole  and takes out two pieces of chewing gum--without their wrappers. Not wanting to spend any more time than necessary on Radley property, she runs home quickly with the gum in her hand. She sniffs it and examines it once she reaches the safety of her front porch. She thinks it looks fine, but she still licks it and waits a while before cramming it into her mouth. At that point she can tell that it's Wrigley's Double-Mint gum.
When Jem comes home and notices her chewing a huge wad of gum, he wants to know where she got it. She divulges that she found it inside one of the Radleys' trees, and he commands her to spit it out immediately. He stamps his foot and warns her that she'll get killed if she even touches a Radley tree. He tells her she must gargle or he'll tell Calpurnia on her, and she reluctantly complies, not wanting to rinse the taste away but fearing Calpurnia's wrath.

In Chapter 4, Scout is passing the Radleys' home and notices a shiny object coming from the knothole of the giant oak tree in the Radleys' yard. Scout runs over to the tree and grabs two sticks of Wrigley's Double-Mint chewing gum out of the knothole. When she gets home, she unwraps the gum and sniffs it to make sure it isn't poisonous. Jem returns home and asks Scout where she got the wad of chewing gum. Scout tells Jem she found it sticking out of the tree in the Radley yard and Jem yells at her to immediately spit out the gum. Scout obeys Jem by spitting out the gum and then informs him she's been chewing it all day. Jem is still under the impression that everything associated with the Radleys is harmful and tells Scout she's not even supposed to touch the Radleys' trees, let alone eat anything she finds in them. Jem insists what Scout did was deadly and commands her to gargle the taste out of her mouth.