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The main reason for this strange phenomenon of  Hydrogen bonding among molecules.

The hydrogen bonding  increases among the molecules of water  when water is cooled from 10 degrees C to 4 deg C.  The hydrogen atoms are attracted by the oxygen atoms in the neighboring molecules.  So the neighboring molecules come closer and closure.

When again water is cooled from 4 degrees C to  0 deg. C the molecules seem to move away from each other, due to the decrease in the strength of Hydrogen bonding.  It is not explained beyond this fact.  It happens only with water.

Thus water has the highest density at 4 degrees C.

In ice below 0 deg C, the molecules move away to form a rigid lattice structure.  So the density of ice is less than that of water.

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