If you dissolve some substances in water you can often get them back again by boiling off the water – a reversible change. But some substances will combine with water to form new, long, thin, needle-like crystals of a new substance. These crystals are too small to see without a very powerful microscope, but as the crystals grow and interweave, they make a strong, hard substance. Once the crystals have grown, the water cannot be removed from them. This is how cement works.
