Which one of those numbers is not like the others 25, 16, 49, 63, 81

Given sets of numbers are 

25 , 16 , 49 , 63 , 81

We can see 25 is a square of 5 because 5 * 5 = 25

Similarly 16 = 4*4 
               49 = 7*7
                81 = 9*9
But we can not write 63 as a square of any number.

SO we can say 63 is not like the others because we can write other one as a square of any number. But 63 is not a perfect square.