So let's suppose we have the simplest alcohol methanol. Now, under acidic conditions, for example, in the presence of some type of acid, let's say HCL. So hydrochloric acid, what will happen is the hydrochloric acid will basically protonate this hydroxyl group. This will basically depart, forming a carbocation intermediate. And then this can act as a nucleophile attacking this carbon either from the top side or bottom side. And ultimately we form a mixture of these two molecules.