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French fries are delicious.

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French fries with ketchup are a little slice of heaven.

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The problem is it's basically impossible

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to pour the exactly right amount.

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We're so used to pouring ketchup that we don't realize

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how weird its behavior is.

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Imagine a ketchup bottle filled with a straight up solid like steel.

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No amount of shaking would ever get the steel out.

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Now imagine that same bottle full of a liquid like water.

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That would pour like a dream.

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Ketchup, though, can't seem to make up its mind.

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Is it is a solid? Or a liquid?

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The answer is, it depends.

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The world's most common liquids like water, oils and alcohols

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respond to force linearly.

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If you push on them twice as hard, they move twice as fast.

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Sir Isaac Newton, of apple fame, first proposed this relationship,

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and so those fluids are called Newtonian fluids.

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Ketchup, though, is part of a merry band of linear rule breakers

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called Non-Newtonian fluids.

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Mayonnaise, toothpaste, blood, pain, peanut butter

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and lots of other fluids respond to force non-linearly.

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That is, their apparent thickness changes

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depending on how hard you push, or how long, or how fast.

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And ketchup is actually Non-Newtonian in two different ways.

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Way number one: the harder you push, the thinner ketchup seems to get.

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Below a certain pushing force,

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ketchup basically behaves like a solid.

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But once you pass that breaking point,

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it switches gears and becomes a thousand times thinner than it was before.

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Sound familiar right?

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Way number two: if you push with a force below the threshold force

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eventually, the ketchup will start to flow.

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In this case, time, not force, is they key to releasing ketchup

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from its glassy prison.

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Alright, so, why does ketchup act all weird?

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Well, it's made from tomatoes, pulverized, smashed, thrashed

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utterly destroyed tomatoes.

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See these tiny particles?

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This is what remains of tomatoes

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after they go through the ketchup treatment.

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And the liquid around those particles?

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That's mostly water and some vinegar, sugar, and spices.

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When ketchup is just sitting around,

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the tomato particles are evenly and randomly distributed.

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Now, let's say you apply a week force very quickly.

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The particles bump into each other,

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but can't get out of each other's way,

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so the ketchup doesn't flow.

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Now, let's say you apply a strong force very quickly.

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That extra force is enough to squish the tomato particles,

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so maybe instead of little spheres,

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they get smushed into little ellipses, and boom!

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Now you have enough space for one group of particles

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to get passed others and the ketchup flows.

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Now let's say you apply a very weak force for a very long time.

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Turns out, we're not exactly sure what happens in this scenario.

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One possibility is that the tomato particles near the walls of the container

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slowly get bumped towards the middle,

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leaving the soup they were dissolved in,

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which remember is basically water,

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near the edges.

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That water serves as a lubricant betwen the glass bottle

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and the center plug of ketchup,

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and so the ketchup flows.

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Another possibility is that the particles slowly rearrange themselves

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into lots of small groups, which then flow past each other.

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Scientists who study fluid flows are still actively researching

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how ketchup and its merry friends work.

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Ketchup basically gets thinner the harder you push,

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but other substances like oobleck or some natural peanut butters

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actually get thicker the harder you push.

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Others can climb up rotating rods,

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or continue to pour themselves out of a beeker,

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once you get them started.

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From a physics perspective, though,

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ketchup is one of the more complicated mixtures out there.

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And as if that weren't enough, the balance of ingredients

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and the presence of natural thickeners like xanthan gum,

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which is also found in many fruit drinks and milkshakes,

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can mean that two different ketchups

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can behave completely differently.

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But most will show two telltale properties:

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sudden thinning at a threshold force,

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and more gradual thinning after a small force

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is applied for a long time.

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And that means you could get ketchup out of the bottle in two ways.

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Either give it a series of long, slow languid shakes

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making sure you don't ever stop applying force,

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or you could hit the bottle once very, very hard.

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What the real pros do is keep the lid on,

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give the bottle a few short, sharp shakes

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to wake up all those tomato particles,

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and then take the lid off

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and do a nice controlled pour onto their heavenly fries.

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