Physics
A hydrogen balloon floats up because of

Air pressure decreases with decrease in height
Weight of the balloon is less than the weight of air displaced by it.
The pressure inside the balloon is more than the pressure outside it
Air pressure decreases with decrease in weight

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

Metals have higher thermal capacity than plastics
Plastics have a lower density than metals
Plastics have a higher thermal conductivity than metals
Metals are good conductors of heat

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