Despite the seeming biological imperative of every dog that's ever ridden in a car to stick its head out the window, it's actually a harmful habit.

Before the 1920s, automobiles commonly lacked windshields. Although cars back then were comparatively slow by today's standards -- the 1920 Ford Mode­l T topped out at 45 mph (72 kph) -- drivers of cars without windshields wore goggles to protect their eyes [source: Automobile Magazine]. Now that windshields come standard on all vehicles, everyone inside is protected from debris and other road hazards. That's great, but you shouldn't try sticking your head out the window going 55 mph (89 kph) down the highway.

With your dog's head out the window, its eyes are exposed to dust, dirt, rocks and anything else that gets kicked up by your car and others on the road. Allowing your dog to stick its head out of the window while you drive leaves your dog's eyes exposed to possible scratches or punctures.

Even if you've gone so far as to purchase goggles for your dog and spent weeks training it not to slide them off with its paw, there are still other dangers that remain when your dog's head is out the window during driving. The soft ear flaps -- called pinnae -- can swell and become tender from flapping in the wind [source: PFD]. The repeated and rapid flapping of the pinnae against your dog's skull from high-speed winds actually causes trauma to the ear; blood pools in the ear flaps, which can cause painful swelling. After repeated trauma, the soft tissue in the ears can scar, causing lifelong problems for your pooch.

Of course, the worst possible outcome from allowing your dog to stick its head out the window while you drive is falling out of the car at high speeds. Dogs can slide through some tight spaces; look no further than the closest home doggie door for proof. A head stuck out of the window can easily lead to falling out the window if you encounter bumpy terrain or some other jarring road obstruction. A spill can cause broken bones, internal injuries and worse if other cars can't brake in time to avoid hitting your pet.

All of this adds up to another conclusion: It's a bad idea to allow your dog to ride in the bed of a pickup truck. All of the dangers of sticking its head out the window from inside the car are increased, since the bed of a pickup offers little or no protection from wind, debris, or falls. On top of that, a pickup's metal bed can reach dangerous temperatures on a hot day, which can cause damage to the soft tissue on a dog's paws. Ultimately, the best place for your dog to ride is secured inside the cabin of the vehicle. Crating your dog or purchasing a special car seat that restrains it like a small child is the best way to make sure you and your pooch arrive at your destination happy and healthy.