Absolute attachment cross sections of single water molecules onto mass-selected protonated water clusters have been measured in the 30-200 size range and for collision energies down to ∼50 meV. The major surprise is that the attachment cross sections are always smaller or equal to the hard sphere cross section. The attractive interaction between water molecules and charged water nanodroplets does not enhance the sticking probability as expected. The attachment probability is reduced due to dynamical processes on a short time scale; that is, inelastic scattering occurs when the duration of the collision becomes smaller than the period of the O**O**O surface bending motion. A simple analytical expression is derived that gives the sticking probability of a water molecule onto a water nanodroplet in a vapor as a function of the vapor temperature and the size of the droplet.