Physics
We always see the same face of the moon, because

It revolves on its axis in adirection opposite to that ofthe earth
It is smaller than the earth
It rotates at the same speed as the earth around the sun
It takes equal time for revolution around the earth and rotation on its own axis

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Physics
A balloon filled with hydrogen will

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

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