If you didn't know it already, the term Btu stands for "British thermal unit". I don't know what gave the Brit's the right to have their own thermal unit, but let's hope they don't ask for it back any time soon, as it is so much easier to comprehend than the metric equivalents using Joules, Calories, etc.
It's common to see the cooling and heating capacity of air conditioners, refrigerators, furnaces, etc., being shown by their Btu ratings or Tons (1 Ton of refrigeration = 12,000 Btu), but what exactly is this Btu? What does it look like? And does The Queen have one?
Simply put, one Btu is the amount of heat required to be added to, or extracted from, one pound of pure water in order to raise or lower its temperature by one degree Fahrenheit. This is about the same amount of heat as given off by a match that's used to light a fire.
So, if we have one pound of water at 100°F and add 40 Btu to it, its temperature will rise to 140°F. If we removed 40 Btu's from our one pound of water, its temperature will drop to 60°F. If you are all following this so far it's not too surprising, as what I've described above is known as "Sensible Heat", and, as you are already reading this, then obviously you are all very sensible people!
Now, if we keep on adding Btu's to our one pound of water, it will keep rising in temperature at a linear rate of 1°F for each Btu added until it starts to boil, which at sea level will be 212°F (100°C).
At boiling point something very interesting happens....