[awalton]

I get that it's fun to try to "big time" me with physical explanations for simple phenomenons that anyone who actually owns a kettle can explain, but it's very simple: the filter isn't there to provide any kind of softening. It's there to keep chunks of limescale from falling out of the kettle when you pour the water.

Yep, that's the big mystery you couldn't solve in 30 sentences. It's a strainer.