I’m five, and I’d like a simple way of understanding water’s triple point.
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If you put water into a vacuum chamber and keep removing all gas, it starts boiling. Boiling takes energy, but there is no heat source. Thus the water gets colder and colder. And it keeps going, because nothing is really stopping water from evaporating/boiling away from its liquid form.
It only stops when the water gets so cold that it freezes! And the freezing point is almost unaltered by (sane amounts of) pressure, so close to 0°C. Now you get some ice forming in the liquid water, but it also is still somewhat boiling. Thus you found a condition where water has all common states at once: solid, liquid, gaseous. This is the _triple point_!
The same more or less works for most pure substances. Stuff gets very complicated if there are mixtures, though.
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