Science Puzzle
Is the Metal Colder Than the Wood?
A steel block and a wooden block have sat side by side on a kitchen table all night. In the morning you touch one with each hand. The steel feels distinctly cold. The wood feels pleasantly warm.
Which block is colder?
The Answer
Neither. After a night in the same room, both blocks are at room temperature, about 21 °C. A thermometer pressed against each would give the same reading.
Objects left together swap heat until they all reach the same temperature, which is called thermal equilibrium. So why does your hand disagree? Because your skin doesn't measure temperature directly. Your fingertip is at about 33 °C, warmer than both blocks, so heat flows out of it into whatever you touch. What your nerves really sense is how fast heat is leaving your skin.
Steel is a good conductor of heat: it carries heat about a hundred times better than wood. It whisks heat away from your fingertip and spreads it through the block, so the skin cools quickly and your brain reports cold. Wood is a poor conductor. It barely draws any heat, so the patch you touch soon warms to nearly skin temperature and feels comfortable.
On a hot sunny day the trick runs in reverse. A metal seatbelt buckle and the cloth seat beside it may be at the same temperature, but now they're both hotter than you, and the metal feels far hotter because it pours heat into your skin so fast.
The principle: Thermal conductivity. Touch senses the rate at which heat flows into or out of your skin, not the temperature of an object, so a good conductor feels colder than an insulator at the same temperature.