Because its molecules can slide around each other, a liquid has the ability to flow. The resistance to such flow is called the viscosity. Liquids which flow very slowly, like glycerin or honey, have high viscosities. Those like ether or gasoline which flow very readily have low viscosities. The following video shows a qualitative test of viscosities.

In the video, the flow of four liquids down four different pipets is shown. From left to right, the liquids are water, rubbing alcohol, vegetable oil, and ethylene glycol. The viscosity of increases as one goes from water to ethylene glycol.

Viscosity is governed by the strength of intermolecular forces and especially by the shapes of the molecules of a liquid. Liquids whose molecules are polar or can form hydrogen bonds are usually more viscous than similar nonpolar substances. Concentrated sulfuric acid, H₂SO₄, is a good example of a liquid which owes its viscosity to hydrogen bonding. Liquids containing long molecules are invariably very viscous. This is because the molecular chains get tangled up in each other like spaghetti—in order for the liquid to flow, the molecules must first unravel. Fuel oil, lubricating grease, and other long-chain alkane molecules are quite viscous for this reason. Glycerol, CH₂OHCHOHCH₂OH, is viscous partly because of the length of the chain but also because of the extensive possibilities for hydrogen bonding between the molecules.

The ordering of the video in terms of these traits make sense. Water is the smallest molecule, and so despite its ability to readily hydrogen bond, it is the least viscous. The main constituent of rubbing alcohol is isopropanol, which retains and ability to hydrogen bond, and is also larger than water. Vegetable oil is made up of lipids, which have long, hydrocarbon chains, and so is more viscous due to this effect. Ethylene glycol, while not as long a molecule as the lipids in vegetable oil, is polar, and has a greater ability to hydrogen bond, due to having two hydroxyl groups. This makes it the most viscous of the liquids tested.

The viscosity of a liquid always decreases as temperature increases. As the molecules acquire more energy, they can escape from their mutual traction more readily. Long-chain molecules can also wriggle around more freely at a higher temperature and hence disentangle more quickly.