Everyone in the building was counting on you to bring a little levity into their lives especially after Harris in 6C was found dead (suicide) so that's why you've hidden all of the building's housecats.
"I simply broke into each apartment that housed a cat and covertly abducted the cat to the hideaway, where all of the cats have been playfully fighting for the past six weeks," you tell the police.
"You should have seen their faces when they all started to realize that every cat in the building was going missing," you giggle, ignoring your lawyer's pleas for you to keep quiet. "They were all like, where are our cats? It was classic!"
The DA is still trying to figure out how to inflate the charges he can bring against you. Since no humans were harmed, it's difficult to get you on anything but burglary and animal cruelty. And since they have to make a deal with you in order to get you to tell them where the cats have been hidden (they're in a medium sized storage unit at the U-Stor-It on Columbus), you're holding all the cards.
"Prank of the century!" you exclaim to the police, cackling with pride. Seeing as the cats haven't been fed in a day, the police are running out of time so they're about to begin beating you in the mid-section with their night sticks.
Happy You Are The Building Prankster Day!