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.