Smoke
It's a foregone conclusion, and everypony knows it but them. It's in the stars, across the land, broadcast in the sky as billowing smoke signals for all to see. There is a fire between them that will not be denied.
They argue about nothing, nostrils flaring, staring each other down with an inch between them. They make up instantly, because it's boring to be apart. They compete furiously just to throw themselves on the ground, panting, sweat-streaked and giddy, and tell each other, hey, you were pretty good, I enjoyed it.
Everypony knows it will happen.
The question is when.