What do you do if you find a wild mouse?

It's probably best to leave it alone. If it's in your house, of course, and you want to get rid of it, you can buy a live trap, catch it, and release it a few miles from your house. (It has to be more than a mile, or it will come right back.) Don't try to make wild mice into pets, though. A mouse that hasn't been held and socialized from a very young age will probably never be truly tame.