How cold, how hot?

Let us put a pot of cold water H₂O on a gas stove.

Let us then stand by it and tell to it as seriously as we can:  "Boil! I tell you, water, boil!"

But alas, nothing happens. The water stays cold no matter how long we stand there huffing and puffing and telling it to boil.

Until we lit the stove and the burning gas starts to move those molecules in the liquid, causing it to heat up and boil.

Celsius
The crucial chemical reaction of heat on water molecules happens at boiling point. Swedish astronomer Anders Celsius (1701–1744) defines a scale where the boiling point of water is at 100 °C. Water freezes according to this scale below 0 at -4 °C.

This detail of water freezing a little below zero is of crucial importance to all life in water everywhere there are freezing conditions. God's great act of creation, this water thing (although the Bible does not tell explicitely that God created water. It is there after the Beginning.)

Fahrenheit
Somewhat unfortunately, Dutch-German-Polish physicist Daniel Gabriel Fahrenheit (1686 – 1736) had already created a thermal scale before Celsius came up with his. This genius had invented two types of thermometers that are not based on the properties of water: one works with alcohol (1709) and the other with mercury in glass (1714), of all the things... Both types are still in use today from meteorology to measuring fever.

According to an article Fahrenheit wrote in 1724, he based his scale on three reference points of temperature. In his initial scale (which is not the final Fahrenheit scale), the zero point is determined by placing the thermometer in brine: he used a mixture of ice, water, and ammonium chloride, a salt, at a 1:1:1 ratio.

This is a frigorific mixture which stabilizes its temperature automatically: that stable temperature was defined as 0 °F (−17.78 °C). The second point, at 32 degrees, was a mixture of ice and water without the ammonium chloride at a 1:1 ratio. The third point, 96 degrees, was approximately the human body temperature, then called "blood-heat"
wikipedia

These two scales are both in use today, Celsius scale about everywhere, Fahrenheit in Calyman islands and Belize. So who cares? The conversion between Fahrenheit and Celsius readings is complicated as water freezes in 32 degrees °F and boils in 212 °F. So why bother?

Nobody would care - except that in addition to these two places Fahrenheit scale is used also in the United States of America.

Kelvin
Fahrenheit and Celsius scales are good for everyday life and give us means to get readings of crucial importance to us, like the heating temperature of the goose in the oven.

In the molecular clouds in deep space it is useful to have a temperature scale not needed in everyday life, where water, ethanol or mercury in glass will do. Not the boiling point of water but the point at which atoms stop moving around.

The Kelvin scale is named after the Belfast-born, Glasgow University engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907), who wrote of the need for an "absolute thermometric scale". Unlike the degree Fahrenheit and degree Celsius, the kelvin is not referred to or typeset as a degree. The kelvin is the primary unit of measurement in the physical sciences, but is often used in conjunction with the degree Celsius, which has the same magnitude. Absolute zero at 0 K is −273.15 °C (−459.67 °F).
wikipedia

We see that it is easier for us humans to grasp "he temperature is near 0 K" than a sentence like "scientists have succeeded in reaching almost the point  −459.67 °F.