This work presents the Wheelie, a computer program capable of detecting and translating facial expressions into commands to control equipment, such as wheelchairs or assistive robotic vehicles, using 3D technology. Every year, degenerative diseases and traumas put thousands of people into situations that inhibit them to control the joystick of a wheelchair using their hands. Most current technologies are considered invasive and uncomfortable such as those requiring the user to wear some body sensor to control the wheelchair. The Wheelie is a solution that does not require the user to wear body sensors, using instead a 3D camera pointing to the user's face. We call this solution as “the mathematics behind the smile” which is able to classify 9 facial expressions in real-time, such as smiles, kisses, and raised eyebrows that are translated into steering commands to the wheelchair (turn right, go forward, and so on). This work evaluates the use of facial expressions to drive commercially available wheelchairs over real life situations. A series of experiments were conducted in order to assess the efficiency of the command acquisition process and the user experience in driving a wheelchair through facial expressions.