There's a reason why "Hey bb, what's your sign?" has been a way to flirt since your grandma's days. Knowing someone's sign gives you a clue into your astrological compatibility—AKA how well you ~get along~ romantically and sexually. If you ask an astrologer about compatibility, they'll want to see your and your boo's entire birth charts. Your Moon, Venus, Mars, and Mercury signs play a major role in compatibility—it's not just your Sun sign.