Physics
On a cold day when a room temperature is 15oC, the metallic cap of a pen becomes much colder than its plastic body, though both are at the same temperature of 15oC, because

plastics have a lower density than metals
metals have higher thermal capacity than plastics
plastics have a higher thermal conductivity than metals
metals are good conductor of heat

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Physics
Conservation of energy refers to the fact that

The entropy of the universe is always increasing
Energy cannot be created or destroyed but can be converted from one form to another
If you conserve energy you will not be as tired
No chemical reaction is 100 percent efficient

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