A marble sits at the top of a very long, elaborate and twisting run right now.
If - and it's a big if - it trundles all the way to the bottom, Andy Burnham might become prime minister.
But hang on a minute, let's not get ahead of ourselves.
That marble might not even set off down the run, or might fly off spectacularly halfway down.
The twists, turns and obstacles standing between the Greater Manchester mayor and a return to Westminster as an MP - a prerequisite for him to challenge Sir Keir Starmer for the Labour leadership - are substantial.
Firstly, Burnham has to decide if he wants to go for it.
On the upside for him, this is a contest close to home, in Greater Manchester.
It is also a contest brought after the departure of an MP on medical grounds. It would be hard for his critics to suggest Andrew Gwynne stepping down is part of an elaborate political plot.
But, if Burnham does say he wants to run, he has to ask the Labour Party's National Executive Committee for permission.
This body, the party's rule book sets out, must give its "express permission" before directly-elected mayors can seek nomination as Labour candidates for elections to the Westminster Parliament.
So the party could turn around and say no - on the grounds that a mayoral election would be expensive and potentially politically tricky.
Plus, frankly, there are those within Labour who would rather he didn't come back to Westminster, such has been, as they see it, the gratuitous nature of his manoeuvring for the top job.
But it is also true that plenty within the Labour movement think he could be something of a saviour for a party and a government that are really struggling.
If Burnham is rejected - the marble flying off the run - what does he do next?
He has a big enough stage, megaphone and profile to do the prime minister real damage, should he choose to do so.
And what happens if he does stand? The by-election would attract vast attention.
Andrew Gwynne won the Gorton and Denton seat comfortably at the last general election, but Labour's popularity has nosedived since.
Reform UK and the Green Party finished second and third respectively, and many suspect they could both be much more competitive this time.
Reform's leader Nigel Farage was quick to relish what lies ahead.
"This by-election is by no means a certainty for Labour; the left-wing Greens will split their vote, and we will be the challenger. We will give it our all," he said.
It is far from certain, even in this scenario where Andy Burnham is the Labour candidate, that he makes it back to Westminster.
But it is also true this might be his one chance to return to Westminster, in a seat in his patch, with a vulnerable prime minister and plenty of chat among Labour MPs about who might replace him.
So finally, let's imagine he is Labour's candidate and he wins.
It is entirely possible he would do so just as Labour did badly in elections in Scotland and Wales, and the local elections in many parts of England in May.
And so he would arrive on those green benches in the Commons at just the point many Labour folk anticipate the prime minister reaches his stickiest moment yet. Is this the point he is a challenger for the Labour leadership, alongside others?
As I say, this marble run is long and twisting. But it could become the political spectator sport of the spring.