One of life’s small hassles is getting ketchup out of the bottle. What does physics tell us about it—and why doesn’t banging the bottom of the bottle help?

It’s a great disappointment to me that ketchup bottles are now made out of plastic rather than glass because now you can squeeze them and you don’t get to appreciate this nice bit of physics. We’ve all been in a pub or a diner, where you have a glass bottle of ketchup. You turn it upside-down, shake it, hit the bottom, and the ketchup doesn’t come out. Then it all comes out at once! [Laughs]

That’s not random. It happens because ketchup has got this weird property known as shear thinning. What that means is, it’s really viscous until you force it to move a little bit. When you’re shaking the bottle, the ketchup can’t go anywhere, so it stays thick. Once you hit it hard enough that it has to go somewhere, then it becomes runny, so a whole load of ketchup comes out at once.