Metal feels colder than wood at the same temperature
Why skin measures heat flow rather than temperature.
Metal feels colder than wood sitting right beside it because skin has no way of measuring temperature directly, it only measures how fast heat is leaving it, and metal draws heat away from a fingertip far faster than wood does even when both objects have spent the whole night in the same room.
A table with a turned wooden top and slim metal legs, left overnight in one room, offers two different temperatures to a hand run across it in a single motion, properly cold at the legs and merely cool at the top, though nothing in the room has ever treated one differently from the other.
Skin senses a flow of heat
Skin contains nothing resembling a thermometer, and no sensor anywhere in it reports back a number of degrees. What it contains are nerve endings that respond to the rate at which heat is arriving or leaving through its surface, and the brain interprets a fast outward flow of heat as cold and a fast inward flow as hot, treating the rate itself as the sensation. A block of metal and a block of wood resting in the same room for long enough both settle to the same temperature as the air around them, since nothing keeps either one warmer or cooler once they have had time to equalise. What differs is thermal conductivity, the readiness with which a material lets heat pass through it. Steel conducts heat a few hundred times more readily than a typical wood, and aluminium over a thousand times more readily.
The moment a finger touches the metal, heat rushes out of the fingertip at a high rate, because the metal carries that heat away from the contact point and spreads it through the rest of the block almost as fast as it arrives. Touch the wood instead, and heat still leaves the fingertip, but far more slowly, because the wood holds it close to the surface. Within a fraction of a second the small patch of wood directly under the fingertip has warmed almost to skin temperature and stays there, since no fresh cool wood is being brought into contact to replace it, while the metal under the same fingertip never gets the chance to warm up locally because the rest of the block keeps drawing the heat away. This is why a hand can tell metal from wood within about a second of contact, long before either object has changed temperature in any way a thermometer would notice.
Standing in water and air at the same reading
Stepping into a swimming pool on a day when the water and the surrounding air have been measured at the same temperature produces two different sensations, and the water always feels colder, sometimes shockingly so. The two agree to the degree on the same thermometer, but water carries heat away from skin far faster than air does. Part of the reason is that water conducts heat roughly twenty times better than air, and part is that it keeps moving against the skin, continually sweeping away the slightly warmed layer next to the body and replacing it with fresh, cool water. Air sitting against skin warms up a little where it touches and then mostly stays put, cushioning the skin from the full temperature difference, while water keeps stripping that cushion away. The table and the pool follow the same rule: the felt cold depends on how quickly a surface can pull heat away, whatever the thermometer says about the thing being touched.
Handles, spoons and tiled floors
Anyone judging how hot or cold something is by touch alone is judging how fast that material can move heat, and that gap between rate and temperature matters most where it goes unnoticed. A metal tool handle left in full sun can grow hot enough to burn skin, yet a wooden or rubber-coated handle beside it in the same sun, at the same temperature, may feel merely warm, because it releases its heat into a touching hand far more slowly and so gives a moment's warning that the metal does not offer. The same reasoning explains why kitchens favour wooden spoon handles and tool designers wrap metal grips in rubber or foam: the covering slows the rate at which heat can leave a hand or arrive from one, which changes how urgent the sensation feels while the temperature of the tool stays exactly where it was.
The effect runs the other way once the surroundings are warmer than skin. In a sauna, where the air sits well above body temperature, the metal fittings are the ones that sting and the wooden benches are the ones that can be sat on, even though both have been soaking up the same heat for hours. The metal is now pushing heat into the skin faster than the wood can, and the skin reports that fast inward flow as heat.
A cold tile floor and a carpeted one in the same unheated room overnight will read the same on a thermometer and feel entirely different underfoot for the same reason. It is also why touch can never be trusted to compare two different materials fairly, since the sensation always flatters the poorer conductor by making it feel closer to body temperature than it really is. Knowing that the feeling measures a rate is the only reliable way to stop being fooled by it.