In spite of its importance for extrusion and molding, the rheology of polymer melts under high shear rates is not fully explained by the classical theory of Doi and Edwards. In this theory, each polymer chain is trapped in a confining “tube.” When the tube is suddenly stretched, the chain follows at first, but it soon shrinks back to its original length (“retraction process”). I propose a different view: Upon stretching, the tube becomes thinner so the chain is squeezed and tends to span a longer length of tube; there is no retraction.