Absolutely nothing, in my view. (Unless it has just been cryogenically treated and the crystal structure is returning to normal.) and what as for physical warm-up, in an interconnect even if very thin gauge, the warm up due to current will be less than the variation in room temperature from day to day. I believe it is smply getting used to it, or ear response changing, or sound memory unreliability, etc.
we are a lot to disagree on that IB. It’s not my experience and i am sure it’s not a mental process.
The same phenomenon of run in for boxes, interconnects, ethernet cables, speakers…
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I’m not disputing you can hear something change; all I’m saying is that attributing everything to burn-in disregards how much we know, scientifically, about how we perceive sound and adapt to changes. If It is something you don’t suffer from, you may just not be human. 
I heard acidity and brightness on the powerblock and power cable i just bought. I listened only 30 minutes and left it run in 20 hours.
Today i started listening again and i don’t hear anymore the brightness and acidity. So how could my brain have the opportunity to adapt to changes? or perhaps, after 20 hours, my ears and brain changed a lot and are immune to brightness now ?
seriously, it’s non sense. This constant pseudo science to deny the audiophile experience is tiring. Sorry.
The same for all ethernet cables should sound the same, bits are only bits, and blablabla…
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That’s one thing that can change, but not the only one. 20hours is a lot of time, so there are plenty of other things that can cause change in your auditory response. Threshold shifts due to exposure, for instance, are not pseudo-science. There are also things like changes in temperature and humidity that perhaps we can notice from time to time. Al I said was that there are many things that can influence this, whereas you insist that there can only be one.
I think if anything here belongs in the pseudo-science category it is the notion of burn-in. Perhaps it does happen, but it is not scientific at least for now. The other stuff is real and measurable.
As for the “audiophile experience”, you are free to believe in whatever you want it, and if that makes you enjoy it more, that’s all that matters. I certainly am susceptible to some of it, and have (for instance) put in a dedicated circuit for the hi-fi and care about cable dressing. I’m just not discounting the possibility that some of what changes is in my head.
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Speakers certainly can change as the compliance of the suspension alters with use, initially it can be quite significant, then very gradual ageing over decades. Some components within electronic boxes may change with heat during initial periods of use, and then might gradually change with age or more drastically at great age. Component values may change with heat, and make a difference to sound between when cold and when at working temperature.
Conceivably, copper (or silver or whatever) wire, or more specifically the characteristics of its insulation if plastic, may change if the wire gets hot enough - such as maybe a very thin speaker cable with very high level playing, or a thin mains cable. But otherwise - especially with an interconnect - day to day or hour to hour room temperature changes will heat change the wire’s temperature more than usage ever will, so if there was to be change it would happen whether if not you even connect it to the system, including in storage before you buy it.
Against that, physiologically ear response can and does change frequently. For an extreme example try clearing your eustation tube by yawning or swallowing, especially if you have a bit of a cold. And then there is tinnitus, from which many people suffer so slightly that they don’t even notice most of the time - but it can change, suddenly or gradually, permanently or reversibly. Then there is the temporary dulling of sensitivity after exposure to noise (which can include music!), and which apparently does not have to be extremely loud for that to happen,
As for psychological effects, they are many and varied. Some people seem to get very upset at the suggestion of a psychological effect, seemingly feeling it is somehow a negative reflection on them - but in actual fact, some psychological effects, like learning to like a new sound, are good good as they ensure happiness, likewise post-purchase justification. I have psychological issue myself, in that I am too readily drawn into music I like and can struggle to compare things if the difference is not marked enough to be obvious.
And then, I can go to a gig and be quite critical of the sound balance at the start, but the band gets going, I enjoy it, and by the end if simply sounds good. Is that the band’s equipment burning in, or a combination of psychological effect on me, with probable dulling of ear response due to sound level?
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Cartridges go through something similar, and you can even observe changes in very repeatable elements like tracking performance.
Agreed
i have experimented burn in since many many years, as a lot of people here. It’s not pseudo science but experience. The pseudo science is trying to deny it because there is not enough knowledge on it .
However i prefer to stop here . thanks. It doesn’t interest me . Try by yourself, or not. Name it as you want .
Effects being physiological or psychological are no less real to you, the person listening - but not everyone will experience the same thing. That is another explanation as to why some people say they hear burn-in and others don’t experience it.
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Much like being swayed by price, if you get more enjoyment out of something because it costs more that is also kind of good value for money.
a lot of verbiage, it’s what my modest brain decodes now. 
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When I installed my MusicWorks block recently, the sound changed dramatically over the first few hours, and then again after a few days. I’ve done enough of these changes to know that it wasn’t in my mind and that the changes were very real. I don’t understand what is happening, nor do I need to, but I’m convinced that these mains products need some use before they demonstrate their full potential.
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exactly the same for me. A big change after few hours and progressive upgrade ( run in) during days.
It is interesting how views as to burn in of HiFi components come into two distinct camps. Frenchrooster believes in burn in because he has experienced it, longer the component is used the more ‘resolved’ the sound will become. He is not alone at least half of the audiophile world believe in the burn in including professional reviewers from all over the world.
On the opposite camp have view of Innocent Bystander that it is our brain getting use to the new sound, or ear response changing or sound, or sound memory unreliability. He is not alone, I found this view often shared by audiophiles who have some connection or experience in designing and making audio components. Some go further describing this burn in as a fallacy and just audio hocus pocus bullshit. Both camps absolutely believe in their views.
In my limited experience I have owned turntables, amplifiers (integrated) also Naim preamp and power amp, Cds, speakers and Akai reel to reel and never experienced this burn in.
Philosophy tackles ‘Levels of Reality’ perhaps burn in should be in cluded and investigated…
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Perhaps “burn in” as nomenclature is misleading or inappropriate. It suggests an alteration that can’t be reversed or taking something and changing it to something else. Perhaps why some dismiss it - as a cable or device will not suddenly change into something else.
Perhaps “Conditioned” is more appropriate. Like when someone conditions their hair, they feel it looks more how it should look.
If you look at a hair follicle before and after conditioning under a microscope, you can see the difference. Less ragged and more smoother !!!
Same with hifi, a new cable might need some time playing to sound and function as it should.
If you were to see the micro crystals within a cable under a microscope you will witness the same smoothing of nano follicles. 
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You say that burn in is not scientific.
What does that mean?
(I have a first class BSc and have studied the history and philosophy of science, and don’t know what you mean by the idea that burn in is not scientific.)
The ‘burn in’ effect is to be expected for mechanical components (cf. a car engine running in).
After power on, a few electronic components do change their secondary properties a little.
Cables (at least ones not operating any where near their RMS current capacity) don’t change at all, however the solder joints do change a little for a short time after formation (and screw connections are never totally stable).
The thing that changes the most by far, is the listener.
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Sorry Xanthe.
When you say that cables don’t change at all you’re wrong.
cables are made of atoms and atoms all change all the time.
Cables stretch and twist and rust and decay and warm up and the coatings around the metal slowly change too in the same types of ways.
So cables are not things, they are processes that are in a constant state of change.
The relevant question here is whether that change is or is not audible.
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