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

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

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Physics
Rainbow is due to

ionisation of water deposits
absorption of sunlight in minute water droplets
refraction and reflection of sunlight by water droplets
diffusion of sunlight through water droplets

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