How does the Joad family get Grandpa to go to California?

The family got Grampa to go to California by putting soothing syrup in his coffee.

Ma said, "Tom, I got a half bottle soothin' sirup I got for Winfiel' when he had them earaches. Think that might work? Use ta put Winfiel' ta sleep when his earache was bad." "Might," said Tom. "Get it, Ma. We'll give her a try anyways. "I throwed it out on the trash pile," said Ma. She took the lantern and went out, and in a moment she came back with a bottle half full of black medicine. Tom took it from her and tasted it. "Don't taste bad," he said. "Make up a cup a black coffee, good an' strong. Le's see—says one teaspoon. Better put in a lot, coupla tablespoons."