The anomalous expansion of water refers to the fact that water, unlike most substances, expands when it freezes. The density of water increases as temperature decreases but reaches a maximum at 4° C then begins to expand.
When liquid water is cooled, it contracts like one would expect until a temperature of approximately 4 degrees Celsius is reached. After that, it expands slightly until it reaches the freezing point, and then when it freezes it expands by approximately 9%.