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

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Physics
Conservation of energy refers to the fact that

If you conserve energy you will not be as tired
No chemical reaction is 100 percent efficient
The entropy of the universe is always increasing
Energy cannot be created or destroyed but can be converted from one form to another

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