Final answer:
A static electricity spark from a doorknob is not harmful because its few hundred watts of power last only a moment, delivering minimal energy. The body's resistance requires more energy to be harmed, and the current of the spark is too low to be dangerous.
Step-by-step explanation:
A spark of static electricity from a doorknob may indeed carry a few hundred watts of power, but you are not injured because this is a very short duration phenomenon. Wattage, or power, equals energy per time (W = J/s), so even a few hundred watts only stands for a split second and therefore delivers a minuscule amount of energy. Moreover, the human body's resistance requires a significant amount of energy to cause harm, which such a spark does not provide.
Also, a spark has a very low current, usually in the microampere range, far below the threshold that can cause harmful physiological effects. It is the current that poses a danger in electrical interactions, not the voltage alone. Therefore, while the spark can be startling, it is not enough to cause injury due to its short duration and low current.