I don't think the following story is supposed to be funny. In fact it is rather tragic. It seems to be included to prove that the brothers were indeed dumb, dumber and dumbest. Of course, when a modern, educated person reads all these stories, they don't see inexcusably stupid people to be judged and ridiculed like their contemporaries did, but rather a trio of men who had lower than average IQs and really needed help they didn't get.
As well as inheriting the farm, the Bakki-brothers also inherited old Brownie, their father’s mare.
Once there was a great storm and the brothers were afraid that Brownie would be blown away, so they gathered up a quantity of rocks and piled it up around her and on top of her, as much as would stay put. After that the mare neither blew away nor indeed ever stood up again.
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