Two twelve-year-old boys were seriously injured when riding on a go-kart. The boys were riding on public streets, ran a stop sign, and were hit by a car that did not see them. The go-kart had clear warning labels on the sides stating that it was only for off-road use only. The manual stated the go-kart not to be used on public streets. The parents sued the go-kart manufacturer. One basic issue in this case would be: