The letters in the highlighted squares are A, B, E, L, L, H, O, C and K. Rearranged, these spell BLACK HOLE.
In general, when something is heated it expands and a reduction in temperature causes it to contract. However, this is not always true. The anomalous expansion (1 Down) of water is an important physical process that allows water-living animals and plants to survive cold conditions. Figure 1 shows how the volume of a small sample of water changes with temperature. As the sample cools, its volume decreases until it reaches 1ml (20 Across) at 4°C, before it starts to expand again. Its density is greatest at 4°C and so water at this temperature sinks to the bottom of otherwise freezing ponds and remains liquid. On freezing, the volume of water increases sharply. Ice (36 Across) has a significantly lower density than liquid water, so it floats.
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