Ketchup is a tricky thing. If you’re dealing with one of the old-fashioned glass bottles of the stuff that restaurants use and want just a little bit for your fries or burger, you’re out of luck. Instead, you get to deal with what engineer Jennifer Segui calls the “all-or-nothing ketchup quandary.” You tip the bottle to try to and pour some out, maybe even jiggle it a little bit, but nothing comes out. So you shake it and tap it, trying to get the ketchup flowing, until suddenly too much comes pouring out too fast, and your food winds up drowning in it.
The reason that ketchup gives you a hard time is because it’s a non-Newtonian fluid. As science video blogger George Zaidan explains in the TED-Ed video below, a fluid like this doesn’t follow the same rules as a Newtonian fluid like water. Its viscosity—a measure of a fluid’s resistance to flow—changes depending on how hard, how long, and how fast force is applied to it.
Left alone in the bottle, ketchup has high viscosity and won’t flow very much or at all when you first start to pour it. Apply a lot of force by shaking the bottle, though, or let a little force act on it long enough by tipping the bottle for a while, and the ketchup’s viscosity decreases and it’ll start to flow, sometimes faster than you’d like.
Watch Zaidan’s video for more on why ketchup acts the way it does and tips on how to get it to behave a little better.