Many pets sleep in the car, but just as many of them wind up getting restless. It might seem fun to let your dog hang its head out the window, but this practice is dangerous. Your dog could be hit by flying objects, and the wind and cold air can cause inner-ear damage and lung infections [source: ASPCA]. Excitable canines can also unexpectedly leap out an open window, leading to serious injury or death.
Instead, bring along a favorite chew toy, treats and other objects that will keep your pet entertained and busy. Stop the car often to let your pet out (on a leash, of course) to stretch its legs and sniff out its new surroundings.