Water exhibits a unique property known as anomalous expansion between 0^\circC and 4^\circC. When water is heated from 0^\circC, its density increases and reaches a maximum at approximately 4^\circC (precisely 3.98^\circC). This means the volume actually decreases slightly in this temperature range. Beyond 4^\circC, the density begins to decrease as normal thermal expansion occurs, and the volume increases with further rise in temperature. This unusual behaviour is why water is densest at 4^\circC.