'Morning star' and 'evening star' name the same thing but don't mean the same thing

Ancient astronomers didn't realize the bright point they saw at dawn and the one they saw at dusk were both the planet Venus. Gottlob Frege used this to argue that a name's reference, what it points to, differs from its sense, the way it's presented to the mind. Two phrases can pick out exactly the same object while carrying completely different, informative meanings.

— Gottlob Frege, Uber Sinn und Bedeutung (On Sense and Reference) — Zeitschrift fur Philosophie und philosophische Kritik, 1892