Fun fact: mixer taps in the UK don't mix! There are two separate tubes, usually one inside the other, for carrying hot and cold water. So, what comes out of the tap is cold on the outside and hot on the inside, resulting in scalding your hands and freezing them at the same time!

...because you can't connect the hot water and cold water systems together (because of aforesaid risk of sucking contaminated water out of the hot system into the cold system).