Physics
The blue colour of sky is due to

Scattering of longer wavelengths of sunlight in the atmosphere
Reflection of sunlight
Scattering of shorter wavelengths of sunlight in the atmosphere
Refraction of sunlight

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

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