Physics
It is easy to burst a gas-filled balloon with a needle than with a nail. It is because

Gas is reactive with the needle.
Nail is more longer than needle.
Nail exerts more pressure than needle on the balloon.
Needle exerts more pressure than nail on the balloon.

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Physics
A dynamo is used to convert

Mechanical energy into electrical energy
Electrical energy into magnetic energy
Magnetic energy into mechanical energy
Electrical energy into mechanical energy

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