Lady bugs?

My cats are going crazy over little bugs crawling on my ceiling.

Lady bugs!

They are cute, and I'm pretty sure they are harmless, but how did they get in?

Just in case you're wondering why there are cat photos on the right, the top one is of Smalls staring at the ceiling. The bottom one is Rocky trying to jump up the wall to get a lady bug.

I'm stumped, but entertained.