The research group including two of the present authors previously discovered a surprising phenomenon in oil flows through constricted channels; at a certain flowrate when there is no cavitation occurring, an infinitesimal cavity suddenly emerges on the wall where the flow separates. The newborn cavity grows and splits releasing many minute bubbles to downstream, as the flowrate increases. To see if this phenomenon occurs in water too, water flows through rectangular and cylindrical constrictions have been meticulously observed with very high resolution in terms of both time and space. As a result it has been confirmed that the same phenomenon does occur in separating water flows as well, although the developing process of a newborn cavity in water flows is different from that in oil flows probably depending on a large difference in Reynolds numbers between both flows. When the flowrate is substantially increased from that for the cavity inception, a large attached cavity is formed on the wall just downstream of the separation point, as it occurred in corresponding oil flows. The pressure inside the cavity has proved to be almost equal to the saturation vapour pressure of water.