I don't know the scope of "nothing" in your statement, but shampoo does help remove dirt and oil, in a way that washing with water only cannot achieve, which is the number one goal of using shampoo for most people.

This is verifiable by observing and touching hair of other people's hair before and after shower, which eliminates the possibility of shampoo manufacturers secretly altering what you perceive with your fingers.

No; you would need to touch people hair after a shampoo shower and after a non shampoo shower to see the difference.

My very possibly wrong understanding is that plain water + the mechanical action of the water being sprayed on the hair + your hand scratching the scalp does a huge portion of the work. Shampoo itself does very little. So if you don’t have any at your disposal; just does “as if”; and for slightly longer and you will essentially be good to go.

Yes, that's exactly what I mean. My girl friend has long hair, and doesn't wash with shampoo every day (which is somewhat common for long-haired people I believe), and the texture after shower is very different.

In college, especially exam week, we had more anecdata. It was possible to see people who 1) had not washed their hair, 2) had washed their hair in a sink with water only, 3) dry-washed with those sprays, and 4) washed with shampoo. It was very easy to tell which they did.

In general, soapy cleaner (or similar substances) is going to help immensely when cleaning oily stuff. And hair can be really oily. Water-only is just not the same.

One scenario I don't have is comparing other soapy products to shampoo. But shampoo aren't more expensive than other soaps anyway, so I never bothered to look.

Washing with soap removes oils. This is a pretty obvious effect from anyone who's ever tried to clean up oil/grease with soap (ex: dawn).

Rinsing with just hot water distributes oils from the scalp down towards the ends and it removes a little bit of oil in the process.

Rinsing with cooler water is less effective at distributing oils but also loses less oil in the process.

Not washing allows grease/oils (and skin/dander) to build up on the scalp and saturate hair near the scalp.

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If you are just rinsing/scrubbing with water, it does a lot relative to not washing at all but there isn't really a comparison when it comes to soap's efficacy at stripping oils/making them semi-water soluble so they can be washed away.

One example - I did ie yesterday shower at gym after workout, after sauna, but didnt have shampoo so just water, cold and warm. Then washed just my hair at home. Hair and skin without any oil in gym, but very different feeling and also behavior of hair when combing. Shampoo makes hair much smoother for example, also less tough / more bendy.

Might I point out that combing and brushing are definitely affected by shampoo and conditioners?

Source: although I am bald now, I had up to a meter of hair at various points in my life.

In total, or in parallel?

Twisted Sister?

Actual soap, with a basic pH, makes hair very difficult to comb.

Huh? It removes oil and dirt.

If I go for a couple days without washing my hair it gets greasy and gross. Water by itself doesn't remove oil buildup.

And yes of course it's like soap. But it's milder. Technically it's a detergent because soap is too harsh.

I've never heard anyone say shampoo is a scam, this is definitely a first...

Well it's not a scam, it's a sham. Don't settle for fakes, demand real poo! :)=

Yeah, i meant as a not changing structure of hair. Open women magazine and read what adverts say their heavenly concoction allegedly does to your hair. It does not do that.