When Brother didn’t think that Doodle was trying to walk, how did Brother try to motivate him?

From the text:

Occasionally I too became discouraged because it didn't seem as if he was trying, and I would say, "Doodle, don't you want to learn to walk?"

He'd nod his head, and I'd say, "Well, if you don't keep trying, you'll never learn." Then I'd paint for him a picture of us as old men, white-haired, him with a long white beard and me still pulling him around in the go-cart. This never failed to make him try again.