<: Strictly For The 'Genius' :--->, page-18

It's a mind game.

The question "Which chicken is different" will have you searching for a single subtle difference, when you find it you proclaim you found which one and stop looking. Different people will see different changes and come up with different answers.

Asking "How is each chicken different" will have you checking out each one for the change.

FWIW I first saw the extra feather in the neck of #6 and thought I'd found it (and stopped looking) then after reading the rest of the replies I realised what was going on.