[Type R]

Carry out his experiment: -

Put some tap water in a clean transparent plastic bottle. Let it sit somewhere where it wont be disturbed/drank for a month or so. Then take a good look at it, preferably in bright sunlight. Don't disturb it too much. You'll see brown 'clouds' at the bottom.

Buy a home filter and perform the same experiment with filtered water. Even after two months, the water will be crystal clear.