Now, what basically happens is an intramolecular nucleophilic reaction takes place in which the hydroxyl group, the oxygen of the hydroxyl group found on the fifth carbon, basically attacks nucleophilically, the carbon of this carbonyl that is part of this aldehyde group, and we form this bond. And that bond is shown in purple here as well as here. Now, why did I draw two different molecules, A and B? Well, because if we examine this reaction, this oxygen can either attack the carbon from the top or from the bottom. And because it has the option of attacking it from these two sides, we form two different isomers. So isomer A and isomer B.