Perspicacious is an adjective that means "shrewd" and "wise." A perspicacious child can't be fooled when her parents try to keep a secret by talking in Pig Latin.
The adjective perspicacious is a long word for a short definition: "keen" or "shrewd." This word is descended from the Latin word perspicere, which means "to look closely." In other words, if you look closely at something you are paying attention to it and know it well. A definition of perspicacious that is out of date but still relevant is "having keen eyesight" and people who can see clearly are also aware and attentive!