Blood Sugar     mmol/L = 18*mg/dl

Consider 12 grams of carbon-12, a particular isotope of carbon.
Count the number of carbon-12 atoms in that 12 grams.
Call it N.

• Collect enough water so that the number of molecules of H2O is N.
The amount of water you've collected is one mole.
• Collect enough iron so that the number of atoms of iron is N.
The amount of iron you've collected is one mole.
• Collect enough electrons so that the number of electrons is N.
The amount of electrons you've collected is one mole.
• Collect enough ...

>Wait! I have no idea what we're talking about?
Okay, here's the deal.
I measure my blood sugar from time to time and I get, for example, 5.9 milli-moles per litre (or mmol/L).
Then I ask myself: "What is a milli-mole?"

>I'll tell you. It's one thousandth of a mole.
And what's a mole?
>Uh ... it's ... uh, N atoms of carbon-12?
See? You don't know either.

A mole counts the number of molecules or atoms or particles (like water or iron or electrons).
Saying "This box contains 3 moles of oxygen" is something like saying "This box contains 3 dozen chocolate cookies".

>But if I take 12 grams of that carbon-12 and count the number of carbon atoms, I'd get a definite number, wouldn't I?
Yes, you'd get N. The number of atoms, N, is about 602,200,000,000,000,000,000,000 or 6.022*1023.
This magic number is called Avogadros number.
If you collect one mole of gas, it'd contain N = 6.022*1023 molecules.

>What kind of gas? Oxygen, methane? What?
Any kind, regardless of its chemical nature and physical properties
If you collect one mole of water, it'd contain N = 6.022*1023 molecules.
If you collect one mole of iron, it'd contain N = 6.022*1023 atoms.
If you collect ...

>Okay! I get it! Uh ... I'll take one mole of chocolate cookies.
I should point out that one mole of iron will weigh more than one mole of water which will weigh more than one mole of oxygen.
I might also point out that equal volumes of gases will contain more or less molecules depending upon their temperature and pressure.
However, if two equal volumes of gas are at the same temperature and pressure, they'll contain the same number of molecules.

>Namely N = 6.022*1023 molecules, right?
Only if the two gases are at standard temperature and pressure.

>Okay, so a mole has N molecules ... or atoms or particles or whatever. How much does it weigh?
Aah, good question. In particular, how much does one mole of sugar weigh? That's the big question, eh?

In fact, if you adopt the U.S. blood sugar measurements (instead of the metric units adopted by most other countries), it's in units of gm/dl and that's confusing so ...

>Huh? What's gm/dl?

As with carbon, there are various kinds of sugar: fructose, glucose, sucrose, etc..
In particular: glucose. (The sucrose molecule is made up of one glucose and one fructose molecule.)
A mole of glucose will weigh 180 grams. (Regular table sugar is about twice as heavy as glucose.)
A milli-mole will weigh 0.180 grams.
A milli-mole of sucrose in a Litre of water will contain 0.180 grams per Litre or 18.0 milligrams per decilitre (1/10 of a Litre).
Write that as 18.0 mg/dl.
Conclusion?

>One mmol/L = 18 mg/dl.
You got it!

>So your blood glucose of 5.9 mmol/L is 18*5.9 and that's ... uh ... 106 mg/dl.
Very clever.