if you add the letters/characters of all 1st word, it will be equal to 21
Isle = 4
Angle = 5
Irk = 3
Otter = 5
Sage = 4
4+5+3+5+4 = 21
then, same to all the 2nd word, add all the letter/characters. and so on so forth.
first words = 21 characters
second words = 18 characters
third words + "one" = 21 characters
fourth words = 18 characters
fifth words = 21 characters
pattern is 21,18,21,18,21