What, exactly, makes a good Eurovision song? To ask anyone backstage at this year's Eurovision Song Contest, Australia's Guy Sebastian has in his hands exactly that.
The problem with trying to make that fit the science of six decades of sequins, big hair and boppy tunes is that the Eurovision goalposts keep changing.
And this year they have changed again.
The competition, once the very zenith of kitsch and Euro-ridiculousness, has somehow morphed into something quite serious. It isn't quite where it wants to be, but it's a good way along the road.
And that means that songs from artists like Sweden's Måns Zelmerlöw, Italy's Il Volo and Russia's Polina Gagarina are musically very powerful in broad terms, overtaking the flimsier Euro-pop that once dominated the competition.
Make no mistake, those songs still exist. And classic Euro-pop touches, like wind machines, are still overused in the competition.
But intriguingly those songs no longer dominate the conversation about who is likely to win and why.
Guy Sebastian's Tonight Again is, like most good Eurovision songs, elegantly simple. Musically, lyrically and in terms of staging, it isn't over-baked. And perhaps that is the trick.
Pyrotechnic touches, seen in the early rehearsals this week, have been abandoned in the final. A very clear decision was made at some point to bring things "in" a little, and it pays off enormously.
It is also helped by a spectacular stage at Eurovision. It may be the butt of jokes from Azerbaijan to Zambia and everywhere in between, but Eurovision has a stage which is the envy of every concert stage in the world.
Tonight Again lacks the immense-ness of last year's winner - Conchita Wurst's anthemic masterpiece Rise Like a Phoenix - but successfully trades that off for a disco beat which makes the song, and its melody, infectious.
There is an old Eurovision recipe which runs roughly like this: anthemic tune, big key change and not too many lyrics. For the most part Tonight Again fits that brief.
Lyrically it also taps the magic and the majesty of Eurovision: a great big party whose participants don't want it to end.
"I don't want tomorrow; tonight's so good," he sings. "This is one tough act to follow; and then tomorrow we can do tonight again."
And if not tomorrow, at the very least next year.
Play our bingo game during the Eurovision Song Contest:
