viscosity
/ vis-KOS-i-tee /
Tip a jar of honey and a glass of water and pour both: water gushes out at once, honey oozes slowly and lazily. That reluctance to flow — how thick or syrupy a liquid feels as it moves — is viscosity. A high-viscosity liquid resists being stirred, poured, or pushed through a pipe; a low-viscosity one slips along freely.
Viscosity is a measure of a fluid's internal friction: how strongly its layers drag on one another as they slide past. When you make one layer of liquid move faster than the next, the molecules grabbing at their neighbors resist that shearing, and viscosity is exactly the size of that resistance. It comes from intermolecular forces (stickier molecules drag harder) and from molecules tangling or interlocking, as long chains in oils do.
Why it matters: viscosity decides how fast blood flows, how engine oil protects metal, how lava spreads, and how thick paint or ketchup behaves. The honest caveat is that for liquids it falls sharply as temperature rises — warm honey pours easily — which is the opposite of gases, whose viscosity rises with temperature. Some fluids (paint, blood, cornstarch in water) even change their viscosity depending on how hard you push them.
Cold honey barely moves when you tilt the jar, but warm it in hot water and it pours almost like syrup — heating loosens the molecules' grip and slashes the viscosity.
Heating a liquid lowers its viscosity and lets it flow freely.
Viscosity and density are different: mercury is very dense but flows easily (low viscosity), while honey is less dense but flows slowly (high viscosity). Note also that liquids thin out when heated, whereas gases actually grow more viscous as they get hotter.