It doesn't happen to him often, so it catches Cisco off guard when he realizes that Sirius is flirting with him, and blatantly at that. He's sure that the expression on his face is 'deer in headlights' for at least a couple of seconds until he blinks himself out of it, closing his mouth and swallowing.

"Straight up," he says, and his voice is a little higher than usual, but he thinks his expression is at least less insane now. "Well, not straight," he adds with a wink, doing his best to flirt back.