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

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

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Physics
The bats can fly in the dark because

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

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