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

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Physics
The blue colour of water in the sea is due to

Scattering of blue light by water molecules
Reflection of blue sky by sea water
Absorption of other colours except blue by water molecules
Reflection of blue light by impurities in sea water

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