The paper deals with the expression "God is able to make a calf from a tree-trunk"—a very popular phrase in medieval treatises, especially in the context of God's omnipotence. Its attestations are thoroughly documented and considered, contexts discussed, and attributions examined. It is argued that the attribution to Anselm of Canterbury is false and late. It is claimed that the phrase goes back to a popular saying as attested by William of Conches, Peter Comestor, and ps.-Bonaventurian Ars concionandi. Thus, it is a rare example of a rustic proverb used in scholastic debates as a standard scholarly argument.