At the present writing, Irish comedian Dara O'Briain hosts Mock of the Week a BBC quiz program where the panelists are themselves largely drawn from the Britain's funny people. Many of the questions are from contemporary news stories, and so the writers are never at a loss for material. Dara also is known for his incredible rapid fire stand-up comedy which among other things allows Americans to learn how the rest of the civilized world sees them as credulous, superstitious, scientific illiterates.
Dara himself is also a frequent guest on Qi, another British comedy quiz program hosted by Stephen Fry. The question on Qi are so difficult that the panel are awarded points on interesting answers rather than correct answers. You even get points if you say something interesting that is only tangential - or even unrelated - to the original question.
Dara once got a couple of points when he elaborated that zero degrees Celsius (or Centigrade) was actually the triple point of water. He said he learned that fact in school and never thought he would have any use for it. Stephen congratulated Dara on his knowledge.
But the following year Dara was a guest again and Stephen asked what was the triple point of water. Naturally Dara said zero degrees Celsius, and the flashing backgrounds and klaxons signaled the obvious but wrong answer. It seems, Stephen said, the triple point of water was zero degrees Celsius when Dara was in school. But in 1994 a new convention was adopted and the triple point of water was defined as "zero point zero nought degrees", that is 0.01 deg Celsius. Dara's earlier answer, it seems, had prompted Britain's more knowledgeable viewers to write in, and Stephen felt he had to rectify the error.
Sadly, Dara's claiming that he was rounding off did not restore the lost points. He then said there must have been millions of viewers who saw his mistake and said "I know it's comedy show but we're not letting the fecker get away with that.
Alas, nothing is perfect not even the admirable Qi. Although the last bit is true - the triple point of water is 0.01 degrees Celsius - the triple point has never been zero degrees Celsius. The original two point definition of the centigrade scale as defined by Anders Celsius himself - as Stephen actually stated - set the boiling point of water at 1 atmosphere as 0 degrees - yes, the boiling point was 0 degrees. The freezing point of water was 100 degrees. It was only the year that Anders died that scientists flipped the definition to our where zero was actually colder than 100.
But even with the later definition, zero was not the triple point. With the zero=freezing and 100=boiling definition, the measured triple point of water is still 0.01 to the nearest hundredth of a degree. So Stephen was wrong when he said in Dara's school days zero degrees was the triple point of water
What happened in 1994 was that both defining points of the Celsius scale were changed. The first - the lowest defined temperature - is now that of absolute zero - the unobtainable temperature at which all molecular motion ceases. It is exactly -273.15 degrees Celsius. The second - upper - defining point of the Celsius scale is indeed the triple point of water and is defined at the number Stephen gave - exactly 0.01 degrees.
The question naturally arises of how the heck can you have a temperature scale where the lowest value - absolute zero - can't be reached. It's very simple, as Captain Mephisto might have said, you simply define the degree as 1/273.16 where 273.16 is the defined triple point of water. What could be simpler?
Oh, yes. These definitions only apply to a very special type of water, Vienna Standard Mean Ocean Water. No, no, this doesn't mean that ocean water from Vienna has a fractious and difficult personality. It's not even ocean water since it doesn't have any salts or impurities. It's simply that you have to specify the isotopic composition of the hydrogen and the oxygen at certain values. After all there's three types of oxygen atoms (with 16, 17, or 18 neutrons) and three types of hydrogen (with 0, 1, or 2 neutrons). The amount of the isotopes effect - although slightly - boiling, freezing, and triple points.
All this means Stephen missed another chance to make the panelists fall into a trap. He could have asked them how are the freezing and boiling points of water defined on the Celsius scale. Everyone would have said 0 and 100 degrees respectively. Then the klaxons and flashing signs would have gone off, and Stephen with much regret would have told them that with the new convention the freezing and boiling points of water are now measured values, not defined. According to the National Institute of Standards and Technology the freezing point of water is now 0.000089 degrees Celsius and the boiling point is 99.9839 degrees.