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Question about Kw, ionic product of water?

I have learnt that in the equation for the ionic product of water the concentration of water is constant. But if that is so wouldn't the concentrations of H+ and OH- also remain constant? Because they are all in one equilibrium and if only a very small amount of the water ionises then only a small amount of H+ and OH- would be produced. So shouldnt they be considered as constants as well.
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Big-Daddy
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If K = [H+]*[OH-]/constant then [H+]*[OH-] = constant*K which is itself a constant, what is hard to understand?

The fact that water's concentration stays constant is an approximation that comes out despite it participating in an equilibrium, i.e. only because that equilibrium is meant to be negligible compared to how much water there is.
In pure water, the concentrations of H+ and OH- can be considered constant, and there, Kw isn't especially helpful. It's when acid or alkali is added to a solution that Kw becomes useful, because even when [H+] and [OH-] change due to the addition of new reactants, still [H+][OH-] will always equal 10-14. It makes a useful way to convert between, for example, the concentration and quantity of base added, and pH.

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