Why a tap can turn ketchup into a rush
Ketchup is not simply a thick liquid. When it sits, tiny tomato particles and other ingredients form a loose internal network that resists movement. In materials science, this is called a yield stress: the sauce will not flow much until a force strong enough to break that structure is applied.
A sharp smack sends a brief jolt through the bottle and sauce. That jolt disrupts the network, making the ketchup less resistant to flow. Ketchup also becomes thinner when it is moving—a behavior called shear-thinning—so once it starts sliding, it can pour more readily than it did at rest.
Why the bottle can suddenly empty
Gravity was already pulling the ketchup toward the opening, but the sauce’s resting structure helped hold it in place. The tap can push it past the point where that resistance is overcome. Afterward, gravity continues the job; if the bottle is tilted and the opening is clear, a small initial movement can become a fast stream.
That is why repeated hard smacks are often less useful than one controlled tap. The goal is to get the ketchup moving, not to launch it. Once the internal structure has been disturbed, the sauce may flow much more freely than expected—turning a stubborn bottle into a ketchup avalanche.
