Steaming (droplet formation) above the surface of liquid water that is being heated can first be observed at about 50 degrees C under normal atmospheric conditions. The droplets have been presumed by previous observers to form on condensation nuclei that are always present due to impurities in atmospheric air. But simple experiments, reported here, show that droplet nucleation apparently can occur spontaneously with warming, beginning at about 50 degrees C, even in purified saturated air at atmosphere pressure that is free of nuclei other than water clusters. The observations suggest new ideas about structure in water vapour. A critical cluster size of about 45 can explain observations over a wide range of temperatures and water vapour partial pressures.