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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