Tom Scott, TV presenter and general creator of awesome things, has taken it upon himself to answer this age-old question.
As he points out, all sorts of ridiculous theories have been espoused.
But what's the truth?
Well, it's very simple. As Scott explains, it goes back to how British houses were constructed after World War II.
Most of them have a cold water storage tank in the attic – it feeds a hot water tank that's for central heating and hot water in the bathroom and kitchen. The water from the hot tank may not be entirely safe.
That cold water storage tank in houses that weren't properly maintained might have been open to the elements, or silted up, or covered with iron rust or – in one particular case you can read about – have a couple of dead rats floating in it.
That water can't be counted as drinking water under the regulations.
In Britain, we DO NOT CROSS THE STREAMS.
Except in modern houses, where there isn't a tank in the attic and a boiler heats up the water on demand.
And that's why if you're in a house with a mixer tap, you can safely drink the hot water.