Viscosity of Liquids:

Several liquids are "runnier" than others. You know there is dissimilarity at room temperature between, say, water and thick molasses. When you fill a glass with water and another glass with an equivalent amount of molasses and then pour the contents of both glasses into the sink, the glass having the water will empty much faster. The molasses is said to have higher viscosity than the water at room temperature. On a tremendously hot day, the difference is less of course than it is on a cold day, if not, obviously, you have air conditioning which keeps the air in your house at similar temperature all the time.

Several liquids are far more viscous even than thick molasses. An illustration of a liquid with tremendously high viscosity is hot tar as it is poured to make the surface of a new highway. Another illustration is warm petroleum jelly. Such substances meet the criteria as defined above to qualify as liquids, though they are thick in fact. As the temperature goes down, these substances become less and less liquid-like and more solid-like. However, it is impossible to draw an exact line between the liquid and the solid phases for either of these two substances. They are not like water; they do not freeze into ice and change state in an apparent way. As hot tar cools, where do we draw the line? How can we say, "Now, this substance is liquid," and then 1 second later on say, "Now, this substance is solid," and be certain of the exact point of transition?