This is regarding the What can I do to prevent static shock? question.
In it, the point was made that what hurts about a static shock is the arc of current through the air? why does the electricity only hurt if the arc makes contact with skin, but if you’re touching a key that makes contact with the arc, it doesn’t hurt?
– Renzo
Surface area. With your finger, only a tiny bit of your skin is carrying the whole current. A lot of electrons moving thru a small crossection will hurt. While holding the key, a lot of your fingers is in contact with the key. The current flows thru a much greater area of the skin surface. Less pain.
The key exists solely to disperse across a larger area of your skin. If you suspend a key by an insulating thread near the knob (whatever) and touch one tip of the key with just a fingertip and then against the doorknob, it will be an ouch.
More a matter of heat. When current travels through a resistive material, it produces a lot of heat. When it travels through a conductor, it doesn’t (or at least, not as much). Air is very resistive, so when the current travels through the air, it gets very hot (this is why you can sometimes see static shocks: The air is so hot it’s glowing). In a finger-to-doorknob shock, your finger is at one end of this bit of superhot air, and superhot air hurts.
When you use a key, the key is still in contact with this hot air, but it doesn’t care. And once the current is in the metal key, it’s producing very little heat, so it doesn’t hurt for your finger to touch the metal key.
Thank you. Both answers make a lot of sense. But on this one about heat, if the air heats up so much that it causes that much pain, wouldn’t it cause a burn on your finger? Or maybe it does, but because the column of air that’s hot is so narrow, that the burn is too small for you to see?
Yep.
Damage to flesh is a combination of things, including heat flux and time of exposure. A very hot flash doesn’t require much time, but a lower heat flux takes more time.
You can see it if you know where to look. A lens helps.
The burns look like tiny white dots.
If you have a VandeGraaff machine then you can jump many sparks to the same place on your finger. That makes the white dot easy to see! 
If it’s a big enough charge, it most certainly does hurt through the key. 
I speak from experience! My dad had the brilliant idea of entertaining us little kids during the intermission at a children’s play by having us all chain up hand-to-hand and shuffle around on the carpet in the really incredibly dry Minneapolis Children’s Theater lobby. Then I reached out with a housekey to touch a metal railing. That was quite a jolt, and we all felt it.