If you are a lover of tomato sauce, you have no doubt had to deal with the very real struggle of trying to get it out of the bottle, particularly if that bottle is glass. If you've ever wondered why you must suffer at the hands of this stubborn condiment, there is an answer: Tomato sauce is a non-Newtonian fluid. Illustration by Lucas Adams.
Click the link below to see the brilliantly illustrated breakdown of the physics behind it, but it boils down to tomato sauce not behaving in a way that "normal" liquids do. Most liquids -- like water or olive oil -- have a constant viscosity, which is the property that describes the resistance of a fluid to flow. These fluids are called "Newtonian fluids", and they come out of bottles in a pleasing manner. Tomato sauce and other non-Newtonian fluids don't play that way:
Non-Newtonian Fluids, on the other hand, have different viscosities depending on external forces. Their ability to flow is dependent on shear rate. Some are shear thickening liquids -- with agitation, they get thicker. Others, like ketchup, are shear-thinning liquids -- with agitation (a thumb on the glass ketchup bottle), they will thin.
This is why you need an external force like a knife or a good solid thump to get it moving, but for a truly hassle-free sauce-dispensing experience (without watery drippage), you should avoid glass bottles altogether and stick to squeeze bottles with a "flower petal-shaped opening" that can "squeeze out a liquid then seal behind it, trapping the remaining liquid and preventing leakage".
Ketchup Physics 101 [Cook's Science]