I have a stainless steel beer mug.
When I pour the first beer; it foams up so much so that I almost can’t pour the whole bottle in.
When I pour the second beer; there’s barely any head at all.
My question is: Why does it foam up on the first but not the second?
FTR, I did a “controlled” experiment. Meaning I did my best to make sure the mug was tilted the same, and the pour rate was the same. (And both beers were the same temp when I poured it.
What are you pouring the beer from? Is the container sitting out opened, or warming up?
Other possibilities: It’s due to some residue of the original drink on the inside of the mug, or it’s due to temperature changes in the mug.
More nucleation sites the first time? Once the mug is coated from the first beer, the second beer’s carbonation can’t find as many nucleation sites to bubble from.
Alternative: the temperature of the mug was brought lower by the first beer, so the second gets a less ‘angry’ pour.
I’m partial to the first explanation.
I’m going to guess its the temperature of the mug. if the mug is warmer than the beer the beer will warm up quickly which will lower the amount of carbonation the beer can hold and you’ll see more foaming. Glass won’t heat up the beer as quickly so at the same temp you’ll see less foaming.
I’d like to ask a related question: why does diet cola foam so much more over ice than regular cola? Situation: take a glass* with ice cubes, pour over a regular cola, and you get a moderate amount of foam. Pour over a diet cola instead, and the foam seems several times as much. This seems to work for the two major brands of cola, and also for root beer (diet vs. regular).
*Doesn’t have to be made of glass, works just as well with plastic or ceramic (mug). I don’t think I’ve tried it with metal.
Third explanation: there was a Mentos in the mug on the first pour.
Yes, but he doesn’t give an answer, he only confirms that it happens. And they didn’t actually try it over ice, they only tried pouring it into a glass; my opinion is that ice makes even more of a difference.
Or are you saying that if the Perfect Master can’t answer the question, no-one can?
Just contributing data to your search for knowledge. And yeah, that was a pretty sloppy Cecil answer.
I’d guess a temperature difference. Warm mug, cold beer. Once the first is poured the second is in a cool mug.
That, or I would guess you have some soap residue left in the mug which reacts with the beer. After the first one it is rinsed off.
So did you do the first pour, take a leisurely time to quaff that beer, then pour another? Or did you do the first pour, pour it out then do the next pour?
My guess would be temp, since the SS mug is a very good temp conductor.
Some other controlled tests:
Temperature: Refrigerate the mug after you finish the first beer to lower the temp again and then monitor the head.
Nucleation sites - try a first and second pour with a different type of mug (assuming the same coating effect would happen on different sub-straights. )
OP: “(If you get this I’ll be impressed”) seems to imply the answer is known. Is it?
Blasphemy! Of course I drank the beer! 
That part was shameless click bait. I’m sorry.
Thanks for the responses guys.
Decades of experience tells me the answer is primarily nucleation sites.
Since I already quoted Cecil once, might as well do the full monty re nucleation: Why do bubbles stream from fixed spots in glasses of beer or soda pop?
This seems very much like the answer to the OP: there’s more available defects in the mug interior on the first pour so gas has more things to latch onto and make bubbles. The second time there’s a coating on the mug interior and fewer sites where bubbles can form = less foam.
I’d expect a glass mug to have generally fewer small defects than a metal mug - whatever the metal, however much the metal is polished - so the stainless mug provides a more dramatic difference on first v second pour than glass.
Do this as an experiment: take your stainless mug and fill it with water and empty it, then do your first pour. Less foam than if you didn’t do that? Case closed.
For your next experiment rinse the mug with tap water before pouring the first beer. I do this with beers that I know are particularly frothy and it really cuts down the amount of foam.
Or use basic observation. Every brewery around here upends your pint glass over a water fountain under the taps before pouring your beer. Otherwise getting it to you would take twice as long and you’d still bitch about the size of the head.
Temp, no doubt. It’s why I like putting (notice I didn’t say “pouring”) or maybe transferring a beer into a frozen mug. It can be done with almost no head at all, if you’re careful! 