Physics
A drop of liquid assumes spherical shape because:

Intermolecular forces are strong in liquids
A sphere has the least surface area for a given volume
A sphere has the largest surface area for a given volume
Inter molecular forces are weak in liquids

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Physics
Conservation of energy refers to the fact that

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

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