When liquid iron loses heat, it will begin to solidify. Molten, liquid iron can be poured into molds, and when the iron cools down and solidifies, the result is a block of iron shaped like its mold (image on the right).

Similarly, when liquid water cools down, it will solidify into solid water. In other words, it will freeze into ice, snow or hail. 

If you could see things at an atomic level, you could observe how the molecules of liquid matter move slowly. When the liquid cools down, the molecules will begin to move slower and slower, before eventually stopping almost completely. When this happens, the molecules will cling on to one another tightly: the liquid matter has transformed into a solid matter. 



Image on the left: Water has solidified into solid ice. Image on the right: When liquid water cools down, the molecules begin moving slower and slower. The molecules will cling on to one another tightly, and eventually stop moving altogether. When this happens, the water has solidified into ice.