A sphinx is a mythical creature with a human head and a lion's body. The most famous sphinx is the Great Sphinx of Giza in Egypt, a massive stone statue that has guarded the pyramids for over 4,500 years. Built by ancient Egyptians, it shows a pharaoh's face on a lion's body, symbolizing royal power and wisdom combined with strength.

The ancient Greeks also had sphinxes in their stories, but with an important difference: Greek sphinxes were usually female and had wings. The most famous Greek sphinx appears in the story of Oedipus, where she guards the city of Thebes and asks travelers a riddle. Anyone who answers incorrectly meets a terrible fate. The riddle she asks is: “What walks on four legs in the morning, two legs at noon, and three legs in the evening?” (The answer is a human, who crawls as a baby, walks upright as an adult, and uses a cane in old age.)

Because of this legendary riddle-asking sphinx, we now call someone sphinx-like or sphinxlike when they're mysterious and hard to understand, keeping their thoughts and feelings hidden. A teacher might have a sphinx-like expression while students try to guess whether they're pleased or disappointed with a project.