You could tell your grandson that water has some amazing properties, without which we wouldn't be here. It has the highest thermal capacity of any substance, meaning that the oceans can hold huge amounts of heat which keeps the whole planet equable. Most substances contract when they are cooled. Water does the same....until it reaches 4 degrees C, when, weirdly, it starts to expand again as it cools down. The upshot of this is that ice is less dense, instead of more dense as you might expect, than liquid water. That's why ice floats on top of your drink. That's also why ice forms on top of lakes in freezing weather. If ice followed the "normal rules" it would sink to the bottom and, eventually, the whole lake would freeze solid and everything in it would be killed. Instead, the ice on the surface actually prevents heat loss from the liquid water underneath by insulation. It's been calculated that if ice were heavier than water and sank, all the oceans of the earth would freeze solid and life on earth would not be possible. Clean water also has a film on its surface made of a single layer of water molecules holding hands. The force is so strong that this invisible, super-thin layer can float a steel needle or a pond skater. This surface tension is vital to plants as it helps water to grip the sides of the tiny tubes in the stem and move up through the plant, without which the plant couldn't exist. Pretty important when you consider how much the earth relies on plant life. Another unusual feature of water is that, unlike other substances, it can easily exist on earth as solid, liquid and gas in huge amounts at the same times. The interchange among the three is what drives our climate. Good stuff is water, and I haven't even mentioned its value as one of the best solvents in the universe.