Obstructing the Movement of Water Molecules Individual water molecules are small—just three atoms, H2O. Left to themselves, they’re very mobile: so water is runny and flows as easily as a stream. (Oil molecules, by contrast, have three chains stuck together, each 14 to 20 atoms long, so they drag against each other and move more slowly. This is why oil is more viscous than water.) But intersperse solid particles or long, tangly molecules, or oil droplets, or air bubbles among the water molecules, and the water molecules can move only a small distance before they collide with one of these foreign, less mobile substances.
Thickening agents in saucemaking are just such obstructing agents. Cooks have traditionally thought of them as binding agents, and this view makes its own kind of sense. The dispersed materials essentially divide the liquid into many small, local masses: and by dividing, they organize and collect it and give it a kind of coherence that it lacked beforehand. Some thickening agents also literally bind water molecules to themselves and so take them out of circulation altogether, and this too has the effect of reducing the fluidity of the continuous phase.