After detention... very late at night...
DETENTION : Cleaning the Quidditch change rooms and showers after a very messy practice on a rainy day. Without magic. Mud everywhere.
James and Sirius, both looking somewhat bedraggled, climbed through the portrait hole into the Gryffindor common room, finding it deserted.
"I'm impressed that you can write your own name so legibly with mud." Sirius told James. "What I want to know is who wrote 'is wonderful' along side it. IN MUD!!"