You already know that water is essential, but that doesn't answer our question. There's something special about water that makes it difficult to imagine life without it and the reason it features in searches for life outside the Earth.
Indeed, NASA's Kepler telescope discovered a potential candidate orbiting around a red dwarf star, roughly 120 light-years away.
The exoplanet with the uninspiring name "K2-18 b" is between the size of Earth and Neptune and sits in the so-called "Goldilocks" zone which is thought to be more benign.
Whether there is life on another planet will likely remain speculation for some time, but on Earth, the evidence is clear.
Your body weight is 60-75 per cent water, and you only need to lose 4 per cent to become dehydrated and losing 15 per cent can be fatal. This means you might survive a month without food, but not likely beyond three days without water.
The water molecule looks like a basketball oxygen atom with a pair of tennis ball hydrogen atoms hanging off the side in a boomerang formation.
The positively charged hydrogen and negatively charged oxygen make it sticky: it wants to stick to almost everything, including itself. You can see this in a droplet where the water hugs itself into a ball.
Water's stickiness (or to be more correct: its "cohesion") makes it extremely good at dissolving other substances.
When you add water to sugar, it wriggles into the crannies between the crystals, breaking them apart. Something similar happens when you put salt in water, separating the sodium and chloride.
This makes water the perfect vehicle for shipping nutrients around the body while carrying wastes away.
Trees use sap - mostly water - to carry nutrients to the top of a tree several hundred metres tall.
But it's not just water working between cells that makes it important. A clue is last week's limp lettuce. The crispiness of a vegetable occurs because each cell is like a little water balloon. Its cells are pumped by fluid pressure and, as water leaks away, the cells become flaccid.
Now you might be thinking, if a bit of water is good, a lot of water must be really good. You've probably heard the story that you need six to eight glasses of water per day.
This is an example of reductio ad absurdum that extends an argument beyond reason.
Your kidneys are not like a driveway that needs constant flushing. Better advice is to simply drink when you are thirsty because your body knows and it will tell you.
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