Oh.

[ She's distracted a moment as she catches up on their thread, and - hah, that's not nice, Bartimaeus, but it is quite funny. ]

Yes, that. He's not made of physical matter. He's a spirit made of the essence of fire and air - which is, of course, movable, shiftable stuff. So he moves and shifts it around. He can look like anyone - or anything - he wants to. Most of the time, he chooses forms that are perfectly tacky and hideous. Honestly, he simply has no taste sometimes.