Buta -1,3 -diene and But 2- yne are

The structure of But-2-yne is given as:CH3--C≡CH-CH3 The structure of Buta -1, 3-diene​ is given as:CH2=CH-CH=CH2 Here, in both of them, the functional group is different but formula is same. Hence, they show functional isomerism.