I was looking through my archive of photos, and found this one that I’d taken of Domino (rat) and Jaci (mouse) many years ago to illustrate the difference between them.

Please note: the ONLY reason I felt it safe enough to take this photo was that I had given Domino (a very sweet rat to begin with) a yogurt drop immediately before, so her mouth was occupied while I held my little mouse, snapped the photo, and separated them right after.

Rats and mice are not friends. Rats are omnivorous; they will eat mice if they feel like it. 

But they’re both great pets!