At the beginning of an explosion the radius cannot follow Taylor's 2/5 scaling, which would lead to a divergence of the front speed. In this fifth episode on the physics of explosions we describe the very early dynamics of explosions, which are often associated with a constant speed built from the opposition between the output of energy and the ejected mass. Eventually, as the front moves forward the mass of swept-up ambient medium builds up until its inertia is dominant over the ejected mass. At this juncture, the density of the ambient medium becomes the main resisting factor facing energy, marking the beginning of Taylor's regime, where the front speed starts to decrease. Only a few flying debris may continue at the ejection speed. For conventional and nuclear explosions, the early regime at constant speed may only take a few microseconds but it can last for centuries for supernova remnants.