Physics
When a bottle of scent is kept open in a corner of a room its odour is felt in all parts of the room. This is due to the phenomenon of :

Evaporation
Sublimation
Diffusion
Vaporisation

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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 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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