Legends state that when someone came upon a Sphinx that person would be asked to answer a Riddle. If the person solved the Riddle they were allowed to pass. If they could not solve the Riddle they would be devoured by the Sphinx.
The Sphinx is also the "Spiritual Guardian of the Mysteries of Nature". Once we solve a mystery it is no longer a mystery. It then becomes new knowledge; new Wisdom. Whenever we solve a mystery (Riddle) we conquer and tame the Sphinx. This is when a new realm, a new dimension, a new Adventure opens itself up to us.
In the Tarot deck the Sphinx sits on top of the Wheel of Fortune, which represents the Cycles of Eternity, and guards the Entrance of a new Cycle. This card is also the 10th card of the Major Arcana. The number 10 represents completion . As one Cycle ends a new Cycle, of a higher degree, begins. The Sphinx, with its Riddle sits at the Entrance of this new Cycle. These Cycles, and the Riddles that accompany them, are eternal.