פירוש על עבודה זרה 60:10

The two baraitot contradict each other—one allows giving uncovered water to animals and one does not. The answer is that the baraita that forbids one to give such water to one’s neighbor’s animal is that it refers to a cat! If one wants to give such water to one’s own cat, it is allowed because this only weakens the cat. But one should not give the water to another’s cat because it might weaken the cat and when his neighbor wants to sell it, it will be in a weakened state.
While I think the particular example is a bit strange (were people really selling cats? What does anyone need a cat for?), the principle that emerges from this source is important. One must be extra cautious when it comes to other people’s property, more cautious than one is with one’s own property.