Does placing a metal spoon in an open bottle of sparkling wine (etc.. .) help keep its fizz?

As a beer brewer, I'm pretty concerned with fizz ;) Since the below may be a little tl;dr, the short answer to your question is, "I don't think so."

This is actually the first time I've heard of the metal spoon "trick", so I can't directly comment on that, but I'll share a little of what I know about carbonation.

Carbonation is carbon dioxide (CO2) that is dissolved in the liquid (beer, champagne, soda). The fizz is that CO2 coming out of solution and escaping into the air.

Keeping it cold helps keep the fizz, as CO2 dissolves more readily into a cold liquid. And conversely, comes out of a warm liquid more readily. Ever pour a glass of warm soda or champagne straight after opening? It'll foam over everywhere.

The only way to not lose all that CO2 is to seal the container. Even then, the CO2 will come out of solution and pressurize the headspace (space in the top of the container). That's why you get a hiss when you first open a bottle.

Again, temperature comes into play. In a warm bottle, more of the CO2 comes out of solution and pressurizes the headspace. Chill that same bottle down, and the CO2 can dissolve back into the liquid, giving you a sparkling beverage again.

Now, back to the spoon. I would think that putting a spoon in the carbonation liquid would produce nucleation points, causing the CO2 to come out of solution faster. This is why many beer glass manufacturers are coming out with laser-etched designs on the bottom of their glasses - the effervescence caused by the nucleation points helps bring out the hop aromas.

My own experience confirms the answers from the related question from Skeptics. If you hang a spoon inside an open bottle, the fizz goes out.

However, I've been able to keep the fizz in the bottle for a couple of days by putting cling film over the top.

Doesn't work. The CO2 gas was dissolved in the soda liquid under pressure. Once the cap is opened the liquid is now at a lower pressure (room pressure) and the gas comes out of solution. That is, it fizzes. Putting a spoon in the bottle does nothing. Resealing the bottle would stop the loss of fizz.

Read somewhere that the spoon cools fastest therefore cools the air in the top of the container. This then creates a 'cold plug' and reduces the amount of fizz that is lost.

It works to me, & I've been doing it continuously for more than 30 yrs- since a friend show me the trick -to keep the fizz put a spoon on top of a bottle, beer, cava, whatever sparkling it works. since ever I wonder why. and browsing out for an answer i fuond you

Still i do not know why it keeps but it does. I suspect it has to do with some geometric issue: the spoon intercepting fitzz waves coming from within the bottle. i don't know