Liquids exist because of attractive forces between molecules. The forces in liquids are not as strong as in solids, in which molecules, atoms or ions are held rigidly in place, but they are much stronger than in gases because gas molecules experience little or no intermolecular attraction. Molecules in the center of a liquid are pulled from all sides by their neighbors. However, at the surface these molecules are only pulled inwards or laterally. There is nothing to pull them outwards. This lateral force is surface tension.
Viscosity is due the same intermolecular forces as surface tension and it is essentially the measure of the internal friction of a liquid, that is, how easily the layers of a liquid drag against one another. In a very viscous liquid this frictional force is strong, making it hard to pour or stir. The viscosity of a liquid decreases with heat. The molecules in the liquid become more excited, meaning the attractive forces between then decrease. (This leads to a liquid boiling when the molecules become so excited that they no longer exert any force over each other.) This is why it is easier to stir a hot liquid than a cold liquid.
The intermolecular forces in liquids are Van der Waals forces or hydrogen bonding. Van der Waals forces are the result of the weak electrostatic interaction between the positively charged nucleus of one atom in a molecule and the negatively charged electrons of an adjacent molecule. Hydrogen bonding is a stronger electrostatic force than Van der Waals forces and occurs between free electron pairs on one molecule and hydrogen atoms of another.
Consider three jugs containing three different liquids. The first jug contains water, the second contains single cream and the third contains thick custard. The water is easier to stir with a spoon than the cream, which is easier to stir than the custard. The water has lowest viscosity and the custard the highest viscosity. Tilting each of the jugs shows the water is the easiest to pour of the three. It is possible to form a meniscus over the spout of each jug which breaks to allow the liquid to pour from the jug. This meniscus is strongest with the custard and weakest with the water. It would also be possible to heat the custard and the cream to make it as easy to pour as the water. This confirms that viscosity decreases with temperature. However, the meniscus formed at the top of the slightly tilted jugs would be weaker, implying that surface tension is directly related to viscosity and also decreases with temperature.