Dear Dr. Boli: What’s the difference between “four-wheel drive” and “all-wheel drive”? I counted all the wheels on my car, and I only came up with four. —Sincerely, Elon Musk.

Dear Sir: You left one wheel out of your reckoning. With four-wheel drive, the four external wheels are powered and move the vehicle. With all-wheel drive, the same is true, but in the event of a loss of traction, the steering wheel will get out and push.