Ketchup, that delicious nectar of a condiment, is more annoying than it should be to pour out and enjoy. Why? Partly because of the dumb bottle it's in but mostly because it's a non-newtonian fluid in more than one way. Watch TED-Ed explain why it's so damn hard to pour out and what you should do instead in this enlightening animation.
Basically, ketchup can suddenly thin once a right amount of force is applied or gradually thin after a long time. So theoretically you should hit the bottle once very, very hard and let time do it's thing or just give long, slow shakes to get it going. But! The best way is to just keep the lid on the bottle, shake it up real short and sharp and then take the lid off. Or you can get an ugly but functional squeeze bottle.