Reid watches it reform with clear interest. He's seen enough actually-magical things by now to have lost most of his wonder, and in its place is his more typical impulse toward scientific observation. Both of the piece reappearing, and of Loki's obvious enjoyment at being able to deny him answers.

Not that he'd expect anything else from a mythological trickster.

"Uh, more often than you might think. I work for the FBI. I don't know if you're familiar with what that is?" He makes it a lilting question to invite an opportunity to explain, if necessary. At the same time, Reid moves another piece on the board.