Jack looks at Rose, and then back to The Doctor. He expected him to know about these things. He reaches to rub the back of his neck, as he goes to explain this to The Doctor.

"It's when there's three people involved sexually." Jack opts to avoid any graphic details. "It's an old 51st century tradition."

Alisa scrunches her face up, and she can't help but laugh a bit. "Poor Jack. 51st century tradition? Uh huh."