[If] one rents a cow to plow on the mountains, and he plows in the valley —- if the colter breaks, he is exempt. [If he rents it to plow] in the valley, and he plows on the mountains —- if the colter breaks, he is liable; to thresh beans, and he threshes grain, he is exempt; to thresh grain, and he threshes beans, he is liable, because beans are slippery.