class Vehicle {
    var currentSpeed = 0.0
    var description: String {
        return "traveling at \(currentSpeed) miles per hour"
    }
    func makeNoise() {
        // do nothing - an arbitrary vehicle doesn't necessarily make a noise
    }
}

let someVehicle = Vehicle()

print("Vehicle: \(someVehicle.description)")
// Vehicle: traveling at 0.0 miles per hour
