For a moment she wondered if he'd protest. It wouldn't have been hard to overcome - implying that he held back out of fear was usually just the trick for getting Sirius over his reluctance. But a moment later there was no need, as Sirius was offering up his hand. It wasn't as though she was going to cut the thing off, no matter how much she may have threatened to when he'd developed a habit for nicking her things.
"Just think, whole swarms of new people for you to terrorize - you must be positively ecstatic," she said absently, folding down his other fingers so that his thumb stood out. Her wand had almost reached the soft pad of flesh when she paused, her gray eyes leaping up to hold his gaze, "You know the rule, don't you? At school, you can get into a much trouble as you like, so long as you don't get caught."
The moment she had finished speaking her wand made contact, pricking a small puncture in his skin. Before he had time to do something like think about jerking his hand away, her grip closed on his wrist. Quickly replacing the wand with the cylinder of silver liquid, she held its mouth just below Sirius' hand. Once a few drops fell from the ribbon of red across his thumb she released his hand.
To let him better see the process, Bella put set the cylinder back down as the liquid inside began to churn like cloudy water. It only took a few moments for the opaque metallic shade to clear completely, puling an air of satisfaction across her smile. Though she doubted he'd be able to sort out exactly what it did, she wondered what sort deduction he might draw.