When pulled along its axis, double-stranded DNA elongates abruptly or 'overstretches' at a force of about 65 pN. DNA's ability to stretch is crucial to its biological function and technological application, and yet the nature of overstretched DNA is disputed. Thermodynamic data from single-DNA stretching experiments do not clearly discriminate between competing physical pictures that disagree on whether overstretched DNA is double- or single-stranded. I present results from a coarse-grained, statistical mechanical model that indicate that the cooperative dynamics exhibited by stretched DNA can be used to discriminate between these competing pictures. I propose experiments to test these predictions.