Fred and George's attempt happened at half past ten on the Saturday morning, in front of a crowd that had gathered with the specific enthusiasm of people who knew that something was either about to succeed impressively or fail entertainingly and were content with either outcome.
He was there. Of course he was there.
They had produced an Aging Potion of genuinely impressive quality — he had checked the brief description Fred had given him with the air of someone seeking a second opinion, and had confirmed that the potion itself was correctly made, which it was. He had not mentioned the age line's mechanism. He had not needed to. If the potion worked, it worked. If it didn't, it didn't.
It didn't.
The line held with the specific indifference of a ward that had been designed by Albus Dumbledore, who had, Ron reflected, spent a hundred and twenty years thinking about the ways people tried to circumvent the things he built. Also the man has been a teacher for teens and preteens for at least half his life, to think he could actually be fooled by something so simple was laughable.
The potion produced two very convincing white-bearded Fred and George Weasleys who crossed the line with enormous confidence and were immediately ejected by it with the specific force of something that had been waiting for exactly this and was pleased to have the opportunity.
The crowd made the noise crowds made when something both painful and funny happened simultaneously.
Fred and George landed in a heap outside the circle with matching white beards of approximately two feet in length, sitting up with the expressions of men who had planned for success and were recalibrating.
'I want it noted,' Fred said, to the assembled students, from the floor, 'that the potion was correct.'
'Impeccable,' George agreed. 'It was the line that was unreasonable.'
Hermione, beside Ron, was laughing in the way she laughed when something was genuinely funny and she had given up managing it. He took a photograph of Fred and George on the floor with their matching beards, which developed into one of the better photographs of the year — the specific quality of two people who had just discovered the limits of an excellent plan and were taking it with the grace of professionals.
He sent a copy to the twins that evening with no note. He received back a drawing of himself with a beard of equal length and a caption that read: 'We know it was you. We are choosing to find this funny. For now.' He pinned it to the inside of his trunk.