Ketchup is really hard to pour — right up until the moment when it isn't and it spills all over your plate. What is it that makes it behave like that?
Even among other non-Newtonian fluids (such as blood, toothpaste, and ketchup's old enemy mayonnaise), ketchup is an odd one. The harder you push on the bottle, the less ketchup wants to come out, but only to a point. Once you pass that point (to be referred to from now on as "the ruined fries threshold"), though, ketchup suddenly stops behaving like a solid mass in the bottle and suddenly becomes a thousand times thinner.
So, what's going on? A lot of ketchup's strange behavior has to do with the grouping of particles within the fluid, as this video from TED-Ed explains. Now, if only physics could also explain the popularity of mayonnaise.
[Via NPR's The Salt]