When we visited some friends in the Netherlands around the year 2000, they happened to show us an old graveyard in their town, and mounted on a post I was thrilled to see an old thermometer that read in °R. That is to say, the Réaumur scale. I had only seen references to it in books. And my friends had never noticed the thermometer and had never heard of Réaumur temperatures. It's simply 80% of the Celsius temperature. That is, water freezes at 0° just like Celsius, but boils at 80° instead of 100°. Wikipedia has a longish article about it, but doesn't explain why Réaumur, or anyone else, ever thought this was a good idea.

I would add two points more: "a hand can bear" (touching hot dry subjects)-- less than 60°C (<140°F) and "boiling of water" -- ~100°C (~212°F)

It took me many years to realize that one of the "horrors" of the dystopian world of George Orwell's <em>Nineteen Eighty Four</em> is that England has converted to the metric system. I was also slow to understand the complaint of the old guy, who wants a pub to draw him "a pint" and complains that "A 'alf litre ain't enough. It don't satisfy." Because... dare I say <em>of course...</em> U.S. Customary units were not all the same as British Imperial units. In the United States, a pint is 0.47 liter, i.e. it is <em>bigger</em> than a half liter. But the British Imperial pint the old guy wanted was 0.57 liters.

We've been officially metric since 1956 but certain interesting glitches still persist. A person's weight is always expressed in kilos but the height continues to be stated in feet and inches. It's the most common practice even though nowadays most people do know their height in centimetres as well. At the same time, human body temperature still continues to be measured and reported even by doctors in °F but everything else is in °C.

Another short story: there are traditionally no leavened breads in India, at least not the kind that uses yeast. That kind of bread was typically associated with the colonial foreigners and it used to sell by the pound. The Hindi word <em style="color: rgb(0, 102, 204);">roti </em>generally stands for "bread" so the foreign leavened bread was called "pound roti", which got abbreviated to <em style="color: rgb(0, 102, 204);">paoroti</em>. It's still called that by the older people and interestingly enough it's still portioned out in pounds or half pounds.

Memorizing some points on the Fahrenheit scale is the mere first step. In my opinion to think in Fahrenheit you need to think about it. To ask yourself questions in Fahrenheit.

What is the range between freezing and boiling of water points?

What is the range corresponding each 10°C?

What are those points of 10°C; 20°C; 30°C; 40°C; 50°C?

What range of air temperature in a place you live in summer (spring, winter)?

How many degrees in 1°C?

What comfortable temperature of your tea is?

And so on, and so forth...