Irish joke

In a convent in Ireland, the 98-year-old Mother Superior lay dying. The nuns gathered around her bed trying to make her last journey comfortable. One of the nuns went to the kitchen. Remembering a bottle of Irish Whiskey she opened it and poured a generous amount into some warm milk. 
Back at Mother Superior's bed, they held the glass to her lips. The frail nun drank a little, then a little more and before they knew it,  she had finished the whole glass.
"Mother," the nuns asked earnestly, "Please give us some of your wisdom before you leave us." She raised herself up in bed on one elbow, looked at them and said:  "Don't sell that cow"