In the base-catalysed dehydration of an aldol, hydroxide ion (a very poor leaving group) is lost. The reason this is possible is that

1. Ait is expelled from the enolate, whose electron pair forms the conjugated C=C bond
2. Bhydroxide is protonated by the solvent to $\mathrm{H}{}_2\mathrm{O}$ before it leaves
3. Chydroxide is a better leaving group than halide ions
4. Dthe C-OH bond ionises first to give a stable carbocation