There's something about tomato sauce (ketchup to our US cousins) that comes out of a glass bottle that just makes it taste better, but getting it out is a war of attrition. This is the science behind the conundrum, and how you can get it flowing.
George Zaidan goes over the physics of it all in this video from the TED-Ed YouTube channel. Zaidan explains that tomato sauce/ketchup is somewhere between a liquid and a solid and that's why it likes to stay in the bottle so much. So how do you get the tasty genie out?
You can either give the bottle a series of slow shakes over a longer period of time, or give it a forceful smack to push the sauce past its threshold. The best method? Zaidan suggests keeping the lid on and giving the bottle a few quick shakes to loosen it all up, then pour. If the bottle is almost empty, however, that may need some extra manoeuvres.
Why is Ketchup So Hard to Pour? [YouTube]