When water is heated from 0⁰C to 4⁰C it contracts continously instead of expanding. Conversely expands when cooled down from 4⁰C to 0⁰C . This expansion of water from 4⁰C to 0⁰C is called anomalous expansion.

When heated from 4⁰C to 100⁰C its expansion is normal like the expansion of other liquids. Since water contracts when heated from 0⁰C to 4⁰C and expands from 4⁰C to 100⁰C , its volume is smallest and the density is maximum at 4⁰C. Hence water is heaviest at 4⁰C.

Reason :

The water molecule, consisting of 2 atoms of hydrogen and one of oxygen forms a Mickey Mouse head-like structure such that the ears are the hydrogen atoms and the head is represented by the oxygen atom. The oxygen atom side of the molecule is slightly negative while the hydrogen atoms side has a slight positive charge. This makes the water molecules drawn towards each other forming hydrogen bonds. Upon freezing, the molecules set themselves in an arrangement that is very open in nature and contains more space than the water in the liquid state. Hence, water is said to expand on freezing and becomes less dense. On the other hand, it contracts on thawing, much unlike most other liquids.
Source

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The normal and anomalous behaviour of water with temperature is demonstrated by an experiment called Hope's experiment and explained below.

Experiment:

A long metal cylinder is taken. It carries a circular trough around it in the middle. Two thermometers are inserted, one near the top and the other near the bottom of the cylinder. The cylinder is filled with water whereas the trough carries a freezing mixture of salt and water.

First the water at the middle section of the cylinder starts cooling. During the cooling process water contrats and being denser falks to the bottom. As the process of cooling(fall of temperature) continues more and more water falls to the bottom. The process of cooling and falling water continues till the temperature of the lower half of water in the cylinder reaches 4⁰C as shown by the thermometer. Thus the temperature of the lower half is at 4⁰C as shown by the thermometer. Further fall of temperature and flowing down of water stops. Instead the temperature of the upper begins to fall because of flow of heat from upper half towards the middle section caused by the difference of temperature. The process of flow of heat continues caused by difference of temperature. The process of flow of heat continues till temperature of whole water in the cylinder attains the same temperature of 4⁰C as shown by both thermometers.

Now the temperature of water at the middle of the cylinder begins to fall below 4⁰C and expands. Being lighter it goes up and so the temperature at the top begins to fall. The process of falling of the temperature and going up of water continues till the temperature of whole of the upper half cools down to 0⁰C. It is indicated by upper thermometer.

Thus as regards temperature the water in the cylinder is divided into distinct parts
(i) one (upper half) with temperature 0⁰C
(ii) the other (lower half) with temperature 4⁰C

The normal and anomalous behaviour of water is also clear from the following graph.

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Effects if Anomalous expansion of water:

1. Anomalous expansion of water is a blessing to marine life. In cold areas like those situated near the poles of the earth the temperature of water in ponds, lakes and oceans starts falling and goes down to 4⁰C in the first phase in winter season. When the temperature of water on the surface falls below 4⁰C, it expands and thus being lighter it does not sink down. Thus the temperature of water at the surface of water cools down to 0⁰C or below forming ice, but under the ice there is dense water at 4⁰. This helps in serving the precious life aquatic animals.
   
   
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2. In winter season the water supply pipes open to the atmosphere ofter burst when the temperature of the surrounding falls below 4⁰C. This is due to the fact that water below 4⁰ expands (anomalous expansion) and exerts pressure to the walls of the pipes and thus causes damage to it.
   
   
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3. In rainy season a lot of water sweeps through numerous cracks and fissures in rocks. In water season when temperature falls below 4⁰ going down to 0⁰C or below, expansion of water take place, thus developing a high pressure. This results in the breaking of the rocks.

Reference:
http://www1.lsbu.ac.uk/water/water_anomalies.html
http://www.wikiwand.com/en/Properties_of_water
https://www.gizmodo.com.au/2013/12/why-cold-weather-makes-your-water-pipes-burst/