How does Mattie try to cure Taylor of her fear of tires?

In Chapter Six, Taylor tells Mattie about her secret fear of exploding tires. Mattie takes a five-gallon can and fills it halfway with water, then throws it at Taylor. She tells her that it is twenty-eight pounds of water, which is the weight in a tire. She says that the damage Taylor just suffered (she just got the wind knocked out of her) is the worst that could happen with the small tires in the shop.