The fish cannot swim and a bird cannot fly but a bird can swim and fish can't fly.
The world they live in is not real. The bird and fish both can understand humans yet they love to fight among them.
They live in a circular home but their earth is an oval shape just like ours.
It doesn't matter how hard you try you cannot find them in reality.
Children love them.
'Bird' and 'fish' are specific to one breed (rather than all birds and fish in their world).
They can also live outside their circular home.