Here's why:

* Monosaccharides: These are simple sugars that cannot be broken down into smaller sugar units. They are the building blocks of more complex carbohydrates. Examples include glucose, fructose, and galactose.

* Disaccharides: These are formed when two monosaccharides combine. Examples include sucrose (table sugar, glucose + fructose), lactose (milk sugar, glucose + galactose), and maltose (malt sugar, glucose + glucose).

* Polysaccharides: These are complex carbohydrates made up of many monosaccharides linked together. Examples include starch, glycogen, and cellulose.

So, while a sugar molecule can be a monosaccharide, a disaccharide, or a polysaccharide, the fundamental building block is always a monosaccharide.