In my des-res, I have two pet rats, Max and Oz. Max was first to move in, and is essentially a rescued rat. His situation was either he moves in here, or he gets fed to a python. He was no older than six months back then, and the thought of him not seeing the next day... well, the decision was a no-brainer for me. Oz was purchased to be a friend for Max, as they're very social creatures; I can't play like a rat can... I value my hands quite a bit.
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There's no fear of them getting dirty, they spend a lot of their lives cleaning and preening themselves... and sometimes each other. And as I live above a shop, there's no concerns of any noise from them.

And there's always a look of intelligence to them, they're always trying to figure something out.