Jem's tears probably fell for several reasons in Chapter 7 of To Kill a Mockingbird, but they all had to do with Mr. Nathan Radley cementing the knothole in the tree that held his secret hiding place. Jem and Scout had been receiving mysterious gifts in the knothole of the Radley tree, but one day on his way to school, Jem noticed that the knothole was filled with cement. When he asked Mr. Radley, Boo's brother told him that the
"Tree's dying. You plug 'em with cement when they're sick."
But when Jem asked Atticus about the tree, Atticus told him that it looked perfectly healthy. Jem was crying because he had been lied to; because his contact with Boo had been silenced; and because Mr. Radley had done it out of plain meanness.