Why does water expand when it cools?

The origin of the density anomaly of water is often explained in terms of the mixture model, in which the low-density ordered "icelike" component dominates by cooling. However, such an explanation based on heterogeneity conflicts with microscopic observations by computer simulation. Actually, heterogeneity in structure exists and a microscopic density fluctuation is observable; still, it is found that the density decreases quite homogeneously irrespective of the differences in local structure. Our finding of two linear correlations, the bond length against temperature and contraction against angular distortion, recovers the density anomaly of water without invoking heterogeneity.