Oranges and lemons smell different even though they are made up of identical molecules. This is due to the fact they exhibit the property of chirality. Chiral comes from the Greek word for hand.
Chirality is the geometric property of molecules in which the chiral molecules are mirror images of each other just like our hands. In order words we can say that chiral molecules exhibit the property of left and right handedness. The mirror images of a chiral molecule are known as enantiomers. Chiral molecules are asymmetric and cannot be superimposed on each other.
Oranges and lemons are made of Limonene molecules which are mirror images of each other.
An orange is just a left-handed lemon. Orange is made of “left-handed” limonene molecule, also known as R-Limonene and lemon is made up of “right-handed” limonene molecule, also known S-Limonene. Thus the mirror image of an orange limonene molecule has the exact same chemical composition, but smells of lemon.