I think this is in Switzerland: If you have a pet that is a social animal, you must own 2 of them, because keeping it solitary would be cruel. That's not terribly strange, but the strange part is the resulting market for guinea pigs, which are popular pets there. People buy two of them, and eventually one dies. So if you buy one more, you have to prove that you already own one and that you originally bought 2 of them. People then get stuck in a loop, constantly buying guinea pigs because it's very rare for 2 to die at once. Thus emerged the guinea pig rental market. You can rent a guinea pig when one of yours dies, so that way you have 2, and when the one you own dies, you return the rented one to the rental agency.