This problem is solved if the halos are a transient phenomenon apparent just after a major cluster merger event (Tribble 1993). This explains the rarity of cluster halos and the properties of the clusters they are found in - there is a very close correspondence between halo clusters and clusters that X-ray evidence indicates have recently merged (Edge, Stewart & Fabian 1992; Watt et al. 1992). The energy available in a merger event is very large---of order the thermal energy in the hot gas---and only a small fraction of this needs to be diverted to create the halo.