Physics
A balloon filled with hydrogen will

Reach a particular height and start coming down
Continue going upwards uninterrupted
Burst after reaching some height
Reach a particular height and remain floating

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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

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 conductor of heat

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