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. They’re then able to make only slow progress, so they flow more reluctantly.