[jplona]

I have studied the properties of ketchup extensively at burger joints all over, and in my expert opinion Wikipedia has it correct here.

This is why ketchup barely flows when you invert the bottle, until you start banging on it and it gets moving and it all comes out at once (thinned by the shear stress of your banging and gravity)

[magicalhippo]

Or you just shake the bottle at high frequency (and low amplitude), and you get a nice flow of ketchup.