Static shocks become especially common in winter because cold-weather air, and particularly heated indoor air, is often dry.
Dry conditions make it easier for electrical charge to build up on your body and remain there until you touch something that gives the charge a fast route away.
That is when the doorknob gets involved.
Static electricity is an imbalance of charge
Matter contains positive and negative electrical charges. Most everyday objects are close to electrically neutral because the charges balance.
But contact and separation between certain materials can transfer electrons from one surface to another.
Walking across carpet, sliding across a car seat, pulling off a sweater, or tumbling clothes in a dryer can all move charge around.
Your body may end up with an excess or shortage of electrons.
That charge can sit there waiting for an exit.
Why does dry air make the problem worse?
Moist air and slightly damp surfaces can provide gradual pathways for charge to leak away.
In dry air, surfaces tend to be better electrical insulators. Charge can remain separated longer and build to a larger voltage difference.
Winter heating makes this especially noticeable indoors. Cold outside air contains relatively little water vapor. Bring that air inside and warm it without adding moisture, and the relative humidity can drop sharply.
Now the carpet, clothes, upholstery, and your body have a much easier time holding static charge.
What happens when you touch a metal doorknob?
Metal conducts electricity well.
If you and the doorknob are at very different electrical potentials, the electric field across the tiny gap between your finger and the metal can become strong enough for charge to jump through the air.
That brief discharge creates the tiny spark and sharp sensation.
It happens extremely quickly, which is why the shock feels like a snap rather than a steady current.
Why can you sometimes see the spark?
A sufficiently strong electrical field can ionize molecules in the air, briefly turning the small gap into a conductive path.
The discharge can produce light, which is the miniature spark you may see in a dark room.
It is the same broad category of physics involved in much larger electrical sparks, although the scale of an ordinary household static shock is very different from lightning or power-line electricity.
Why do clothes cling in the dryer?
Clothes constantly rub and separate as they tumble. Different fabrics can exchange electrons, leaving pieces with opposite charges.
Opposite charges attract, so socks and shirts may cling together.
The dry, warm environment inside a dryer is almost an advertisement for static buildup.
Can humidity reduce static shocks?
Often, yes. Raising indoor humidity to a reasonable level can help charge dissipate more gradually. Moisturizing very dry skin and using antistatic products can also reduce some everyday static problems.
The goal is not to eliminate electricity from the house. That would create several larger inconveniences.
You are simply giving stray charge a less dramatic way to leave.
So the winter doorknob shock is not the knob suddenly becoming electrical.
You built up the charge along the way.
The knob just handled checkout.
A related explanation is Why does metal feel colder than wood?, which looks at another familiar effect from the same general system.
And that’s the short explanation.
Related explanations: Why does metal feel colder than wood? and What is pH?