They have been serving your guests all evening. Topping up glasses, clearing plates, apologising for the wait on the mains. Utterly unremarkable, right up until one of them drops a tray.
Then there is a row. Voices raised, a manager brought over, the whole room shifting awkwardly in their seats and staring at the tablecloth. It goes on just long enough to become properly uncomfortable.
And then the singing starts, and the penny drops all at once.
What follows is a full show from performers who have spent the last hour learning exactly who is in the room. It is the single most reliable way to turn a nice event into the one your guests are still talking about years later.