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.
Needle exerts more pressure than nail on the balloon.
Nail exerts more pressure than needle on the balloon.

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Physics
In a railway track, two rails are joined end to end with a gap in between them because

Steel can be saved
Air-gaps are necessary for bearing the weight of running train
Accidents due to expansion in summer can be avoided
Accidents due to contraction in winter can be avoided

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