why is jem offended by scout , and what does he do for revenge?

jem is telling stories.... trying to come up with a new game to play and frighten Dill with superstitions at the same time. Scout verbally reprimands him. 

“What’s a Hot Steam?” asked Dill.

“Haven’t you ever walked along a lonesome road at night and passed by a hot place?” Jem asked Dill. “A Hot Steam’s somebody who can’t get to heaven, just wallows around on lonesome roads an‘ if you walk through him, when you die you’ll be one too, an’ you’ll go around at night suckin‘ people’s breath-”

“How can you keep from passing through one?”

“You can’t,” said Jem. “Sometimes they stretch all the way across the road, but if you hafta go through one you say, ‘Angel-bright, life-in-death; get off the road,

Don’t you believe a word he says, Dill,” I said. “Calpurnia says that’s niggertalk.”

Jem got his revenge when he got Scout in the tire and pushed with all his might.... she landed at the foot of the Radley's front porch. 

Until it happened I did not realize that Jem was offended by my contradicting him on Hot Steams, and that he was patiently awaiting an opportunity to reward me. He did, by pushing the tire down the sidewalk with all the force in his body.