Physics
The sky appears blue because

The atmosphere scatters blue colour more than the others
In white light, the blue component dominates
All colours interfere to produce blue
It is actually blue

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

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