It was that January that Scorpius suggested the Seposita Stilus charm. Rose hadn't known what it was, until he'd explained.
The charm would be cast on two pieces of paper, allowing each to see what the other said. He could write a message on one, and it would show up on the other. She could turn it over, write back, and that reply would show up on his copy. They'd be able to wipe the previous messages off before writing new ones.
Once he'd described it, Rose realised that she had heard of the charm before. It was something her older cousins had mentioned using to pass notes in class, and something her uncle George had experimented with, to sell in the shop. The way he'd talked of using it made her think of text messages, something she'd only recently learned about when her muggle grandmother had let Rose play with her mobile phone.
They'd tried it at once, writing messages to one another all through potions. It was almost pointless, since they were sitting right next to one another, and they nearly got detention when the professor realised how little attention they were paying, but it was fun, and quite addictive.