Physics
A hydrogen balloon floats up because of

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

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Physics
Lightning is produced when

Similar charges of electricity rush towards each other and then get repelled
Clouds strike against impurities in air and the friction burns up these impurities
Strong opposite charges in different clouds break down the resistance offered by the intervening air
Water vapour produces electricity in the clouds

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