Why do you have to tap a ketchup bottle?

Ketchup is packed with finely ground bits of tomato. When those bits touch and link up into a loose network, that network resists flowing, so tilting the bottle is often not enough. It is the same reason a squeeze of ketchup stands in a little mound on a plate instead of spreading out like water.

Scientists call ketchup a soft solid: it barely moves until the push on it passes a certain threshold. If the weight of ketchup in the bottle cannot get past that threshold, it stays put even upside down.

Shake the bottle or give it a sharp tap, and the tomato network breaks up in an instant. The bits gain room to slide and line up along the direction of flow. Once ketchup starts moving, the faster it flows the thinner it gets, which is why a trickle can suddenly turn into a gush.

Scientists call getting thinner under force shear thinning, and thickening back up over time at rest thixotropy. So you get a short window right after shaking, but leave the bottle standing and it firms up again.

The usual explanations are that the opening is clogged or that air cannot get in. Both are partly true. Dried ketchup in the neck really can plug it, and for sauce to leave through a narrow opening, the same volume of air has to get in. Giving air a path can help a bit.

But even with the cap off and the opening wiped clean, ketchup will sit inside an upside-down bottle. The key is the strength of the ketchup itself, and shaking or tapping is how you push it past that strength.

First, shake the bottle hard a few times with the cap on. That remixes settled solids and any watery layer on top. Then open it and tilt it to about 45 degrees. Hold the neck with one hand and tap the bottom or the neck lightly with the other. Some glass bottles even mark a spot on the neck for tapping.

Start gently and build up, because a hard first whack tends to send out a flood. When little is left, cap it, hold it upside down, and swing it down sharply and stop, so the ketchup slumps into the neck.