Tattoos

Sirius is bored and Remus just wants to read. But when they get upstairs to the dorm, suddenly Sirius has something to show him. Remus is surprised, but he's also amazed at what's on Sirius' back. But he's not a surprised as he is later that night when Sirius asks him a question. So, what does Remus say? And how down Lily, James and Peter react when they see it?