Sirius had just been passing through the park, no particular destination in mind except for the one he was somewhat avoiding (and it was on the other side of the city so that worked for him). Everything had seemed normal when he entered it.

At least until he started noticing people disappearing and reappearing, and almost walked into a tree himself. Sensing that this was no ordinary prank, Sirius looked around for who might be behind it and recognizes the man from the alley. He's not that hard to spot really.

"Alright," he asks, wandering up, "How are you doing it?"