At dusk one evening, as X was hauling a load of rock in his truck, he saw a large piece of rock fall from the truck onto the travelled portion of the roadway. X continued on his way without stopping. Y, who was travelling in her automobile along the same road some time later, collided with the rock and damaged her automobile. Discuss the liability of X.

X, a trained and licensed plumber, carelessly installed a steam heater in Y's restaurant, and, as a result, Y was seriously burned when a heater pipe exploded. Z, a qualified medical practitioner who treated Y at the hospital, prescribed an improper medication as treatment for the burn, which aggravated the injury. This obliged Y to undergo an expensive skin-graft operation to correct the condition. Discuss the responsibility of X and Z to Y.