Logic Puzzles: Knights and Knaves

There are some fun (and often frustrating) logic puzzles that involve knights and knaves.  Knights always tell the truth, and knaves always lie.  Here is a sample of a puzzle:

There are three people (Alex, Brook and Cody), one of whom is a knight, one a knave, and one a spy.

The knight always tells the truth, the knave always lies, and the spy can either lie or tell the truth.

Alex says: "Cody is a knave."
Brook says: "Alex is a knight."
Cody says: "I am the spy."

Who is the knight, who the knave, and who the spy?

This comes from the website  http://www.mathsisfun.com/puzzles/knights-and-knaves.html (which includes a solution).  The website is a great source for other logic puzzles.

Here is an image that relates to work we have done in class.  Patrick says "I am a knight and Quin is a knight".  Quin says "Patrick is not a knight".  The first row of the table would be if they are both knights, but then this contradicts what Quin says.  The second row is when Patrick is a knight and Quin is a knave.  This row contradicts their statements.  The third row is when Patrick is a knave and Quin is a knight.  This does NOT contradict their statements.  The fourth row is when they are both knaves, but then Quin's statement would be true which is a contradiction.  So only the third row works, Patrick is a knave and Quin is a knight.



Knights and Knaves puzzles are so popular that they even have their own wikipedia page:
http://en.wikipedia.org/wiki/Knights_and_Knaves

Here is a video showing how to solve a knights and knaves problem:
https://www.youtube.com/watch?v=tQr7D92Plck

They are great ways to frustrate your friends :)