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

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

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Physics
The bats can fly in the dark because

They generate flashes of light
They can see the objects in darkness
They generate ultrasonic sound waves
They have weak legs and are likely to be attacked by predators

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