He had been about to tell her that if she kept her things in order like he did then she wouldn't lose everything. He didn't ever lose anything. Though that wasn't really true. He was to excited to get inside the box, he could help Narcissa put her things in order later. Regulus reacher out and picked up the box be very careful with it.
He opened the lid and tipped it so that Narcissa could see what had been scurrying around inside. Three little mice happened to look scared out of their mind when the sunlight hit them. One even started to squeak softly. "Lycoris caught them." He wouldn't dare touch them, they were dirty creatures no matter how many times he had used a cleaning charm on them. They were the only ones that had lasted over winter break. The other two had died even though he put plenty of bread in there to last them.
"I am going to keep them. Once I get enough I am going to get a second year to let them loose in the Gryffindor common room." Regulus normally wasn't one for pranks, and this one most likely wouldn't even get done but it was high time the Gryffindors got a taste of their own medicine even in thought.