To Celeste Tyler and her teenage friends, text-messaging is as effortless as tying their shoes. The high school senior can text without looking, sending messages on her red Samsung “slider” while it’s behind her back, in her purse or under her desk at school, where cell phones are banned. So why not do it while driving? Well, now the law. A state rule now in effect prohibits 16- and 17-year-olds from using any device to talk or text while driving, except in an emergency. A companion law allows adults to continue chatting away, but says they must use a hands-free device while driving.