"To say" is a transitive verb; "To tell" is an intransitive verb.
As a transitive verb, "to say" requires that you specify in the same sentence what is being said. You wouldn't write "I say." You would write "I say ______."
As an intransitive verb, "to tell" does not require you to specify You can write "I tell."
"Imagination" and "fancy" mean approximately the same thing, though "fancy" often suggests an element of spontanaeity or whim. Very loosely speaking: ghosts and goblins hiding under your bed is a product of imagination; suddenly deciding to buy yourself a cherry-red Mercedes Benz is a product of fancy.