Membrane fusion is a key process for cell growth and intercellular communication. There are many models for fusion with widely divergent activation energies. Surprisingly, no comprehensive quantification of fusion was ever experimentally performed. Probably this is because of the difficulty of observing and quantifying rare spontaneous fusion events and equally the difficulty of establishing that such events are bona fide fusion events. Here, we find that the activation energy is lower by far than in most predictions. The biological importance of this low energy value is that it explains how cells can maintain traffic among distinct compartments without mixing them up, preventing spontaneous fusion but allowing specific delivery of cargo as soon as fusion-inducing proteins are in place.