Question: Why does tapping the 57 on heinz ketchup work to get it out of the bottle?
About the article "Ketchup", section "Viscosity".

Candidate passages:
Tomato ketchup has an additive, usually xanthan gum, which gives the condiment a pseudoplastic or "shear thinning" property.
This increases the viscosity of the ketchup considerably with a relatively small amount added - usually 0.5%.
- which can make it difficult to pour from a container.
However, the shear thinning property of the gum ensures that when a force is applied to the ketchup it will lower the viscosity enabling the sauce to flow.
A common method to getting ketchup out of the bottle involves inverting the bottle and shaking it or hitting the bottom with the heel of the hand, which causes the ketchup to flow rapidly.
A technique involves inverting the bottle and forcefully tapping its upper neck with two fingers (index and middle finger together).
Specifically, with a Heinz ketchup bottle, one taps the 57 circle on the neck.
This helps the ketchup flow by applying the correct shearing force.
These techniques work because of how pseudoplastic fluids behave: their viscosity (resistance to flow) decreases with increasing shear rate.
The faster the ketchup is sheared (by shaking or tapping the bottle), the more fluid it becomes.
After the shear is removed the ketchup thickens to its original viscosity.
The relatively high viscosity and pseudoplastic nature of ketchup can make pouring it from a glass bottle somewhat difficult and unpredictable.
According to the Heinz trivia website ketchup exits the bottle at a speed of .
The website also notes that hitting the "57" mark on a glass Heinz ketchup bottle makes the ketchup pour out more quickly and easily.
The "New York Times" also notes that Heinz placed the "57" mark on that particular spot of the bottle as the best place for consumers to hit for the ketchup to pour smoothly.
The Heinz website claims that only 11% of consumers know this fact.
However, while the number "57" is used for the function of pouring, the number "57" resembles all the different types of products that the company offered.
The company actually manufactured 60 products when Henry Heinz introduced the number "57" on his ketchup bottles, Henry thought the number "57" was lucky and continues to use it today. 
The relatively high viscosity and pseudoplastic nature of ketchup can make pouring it from a glass bottle somewhat difficult and unpredictable, and several urban legends surrounding this phenomenon have arisen.
According to one popular folk remedy, repeatedly hitting the "57" mark on a glass Heinz ketchup bottle makes the ketchup pour out more quickly and easily.
The "New York Times" claims this is a matter of intentional design, with Heinz having placed the "57" mark on that particular spot of the bottle as a target for consumers to hit.
Whether or not Heinz intended the 57 as a target is unconfirmed; however, the effectiveness of this method of ketchup disbursement has been empirically confirmed as universally effective.
Simply hit the 57 firmly with the palm of your hand in order to free the flow of ketchup. 
In Collinsville, Illinois, the largest ketchup packet was created by H. J. Heinz Company for a fundraiser for the Collinsville Christian Academy.
People could buy a bottle of ketchup for $1 to add to the ketchup packet.
After it was filled,it weighed 1,500 lbs.
and it was tall and wide. 
 Heinz 57, a brand of sauce, and the number of varieties of foods claimed to be produced by the H.J.
Heinz Company "Prop(osition) 57", one of a number of anti-ketchup packet groups on Facebook designed to bring attention to the shortcomings of take-out condiment packaging; its name is a reference to Heinz Co., which debuted a new design in test markets in early 2010 A fast food dinner in Pereira, Colombia