I don't have any ferret myself, but one of my friends do. He had a pair of caged ferrets. I was wondering why they require cages all these years. He told me once he let them get loose in the house, one of them got into the wall and took him forever to get it out. I don't think they are little submissive pets as you might think they are.  If they have the chance, they will get out of the house.
Those litter trained ferrets must have been trained during very young age. If yours aren't that young any more, it will be hard to do the same.
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