A sphinx is a mythical creature with a lion's body and a human head. The original sphinx of Greek mythology also had wings. The most famous statue of a sphinx is in Giza, Egypt.
The Great Sphinx of Giza is the one you've probably heard of — it's near the pyramids on the banks of the Nile River. Most sphinxes were made to represent various Egyptian pharaohs. The original Greek sphinx told riddles and ate those who couldn't solve them, until Oedipus finally got one right and that sphinx died. In Greek, sphinx means "the strangler.” If someone is like a sphinx, it just means they’re mysterious and quiet, not that they’ll strangle you.