Like I understand it’s reflecting heat, cool. But like with the direct heat when you put it in the oven, how does it not just get hotter?
In: Physics
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It does get hot. But since it is so thin, it has very little mass and a very high surface area compared to that mass. So by the time you’ve pulled it out of the oven, it’s already cooled down quite a bit. That very low mass also means there is not much actual heat energy stored there to burn you.
It does get hot mainly by convection (hot air transferring heat to the foil) but unlike a thick chunk of steel it has very little mass to retain that heat. When you touch it the area you touch will rapidly cool down due to conduction with your finger.
Everything in the oven gets hot. But because tin foil is so thin, when you open the oven and the air hits it, it gets cold really fast.
Metals “conduct” heat, they get hot or cold relatively fast. A piece of glass (cookware) of the same thickness may stay hot for a while; it takes a while for it to deliver all that heat energy to your finger.
How much heat energy is delivered to your fingers depends on the thickness (mass) of the material, and on its [heat capacity](https://en.wikipedia.org/wiki/Heat_capacity). Water has high heat capacity, it takes a lot of flame to get it hot, and it’s not even red hot. Tin foil, very little flame will get it red, yellow, white hot.
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