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Comment by BobaFloutist | Hacker News Reader

Probably my favorite kitchen experiment has always been to put water and pepper into a small bowl, and then dip a toothpick with dish soap into the middle, causing all the pepper to instantly and dramatically scatter away from the toothpick.

YMMV of course, but I always felt that it demonstrates something that's not all that visible and obvious otherwise - I feel like a lot of kid-friendly science experiments give an expected or intuitive result, whereas soap's weakening of intermolecular forces in such a dramatic and sudden fashion manages to connect the micro world with the macro world in a clearly visible, almost shocking way, while neatly demonstrating both why water is special and why soap is helpful.

I dunno, I just always loved it.

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I always enjoyed the corn starch liquid or solid test where the mixture behaves like a liquid while being stirred but behaves like a solid when sitting still.

Non-Newtonian fluids are fun... until they aren't.

One time a group of college students I hung out with got a bulk order of silly putty. This was fun to play with. Until someone had the idea of dropping it from about 6 feet up onto the floor. It shattered. Picking up slivers of silly putty from the carpeted floor was not fun.

Elsefluid ... a different category (shear thinning - https://en.wikipedia.org/wiki/Shear_thinning ) has it so that if you drop something through it, it takes {some time}. If you immediately drop another thing through it at the same spot, it takes {some less time}. One of the chemistry professors wanted to do some experiments with this type of fluid and had his grad students develop a proof of concept. There are some challenges with this - it can't be a thin cylinder of the fluid you drop it through since the distance to the walls is a variable in the speed too. It also needed to be a tall enough cylinder with enough viscosity that you could time it with a stopwatch (initially). One such fluid that has this property is ketchup. They bought 10 gallons of ketchup from the local grocery store (that's all that was on the shelf ... seriously, you need about 7x 20 oz bottles to make one gallon), poured it into a large cylinder... realized that they needed a clear bottom to see it, made a new cylinder with a plexiglass bottom, and did the experiment. They dropped steel ballbearings from height into the the ketchup and timed the duration it took to drop one and the same spot that another had just traveled through vs a new spot. It worked and so they then worked on synthesizing 40 liters of a particular polymer that had that property (another significant effort - took much longer than buying 70 bottles of ketchup) but had the advantage that you could see through it.

> Picking up slivers of silly putty from the carpeted floor was not fun.

One of my science teachers in high school told me about the time she got a chunk of sodium and a 50 gallon barrel of water. She threw the sodium into the water. It blew up as expected. But it also fragmented into a million pieces, and those pieces hit the dewy morning grass and also exploded, triggering a chain reaction. I think she said the students had to run around and stomp out a bunch of little sodium fires. That demo was not done the next year.

> if you drop something through it, it takes {some time}

This is totally unrelated to your story, but it reminds me of the demonstration where you take a strong magnet and drop it through a copper tube. It takes forever to reach the bottom (because the moving magnet induces a current in the copper pipe, which produces a magnetic field in the opposite direction). Honestly, it's probably the one science demo that blew my mind when I saw it. It's SO slow. And we had already learned about the math behind it, but was amazing to see in practice.

> Honestly, it's probably the one science demo that blew my mind when I saw it. It's SO slow. And we had already learned about the math behind it, but was amazing to see in practice.

Many years ago (searching... Amazon 2017) got a fidget toy that did that... and digging around, its https://www.moondrop.space (It was $25 when I got it (for the set of three)... I wouldn't spend the current price on it).

From their website:

> Moondrop Lunar imitates the Gravity on the Moon.

> Moondrop Mars imitates the Gravity on Mars.

> Turn this Moondrop into Earth version simply by removing the magnets from inside.

I was wondering if they were just going to sell the last one without any magnets for the same price. Imagine the profit margin from people just wanting a complete set!

A truly amazing number of spelling errors per page.

Agreed it is a great demonstration, and I remember doing a similar one with an aluminum foil boat as a kid. Make a boat out of foil, put it in water, then add a few drops of dish soap behind the boat and watch it "sail."

The scientific name for this is the Marangoni effect [0].

0: https://en.wikipedia.org/wiki/Marangoni_effect

The „Ölfleckversuch“ („oil stain experiment“) is very similar in setup but you use a drop of oil instead of detergent. Its volume and the covered area on the water surface are used to estimate molecular sizes:

https://de.m.wikipedia.org/wiki/%C3%96lfleckversuch

You might want to run this through your favorite translation service since there does not seem to be an English article on it.

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