There are many resources available online for this lesson plan. I used this for kids 7 and up. It was also a great refresher for me. I remember learning the Pythagorean Theorum in school, but I was never told why I had to learn it or why it was important to know. (Clicking on the picture above will take you to a free online text of the book.)
Read the book, What's Your Angle, Pythagoras and discuss. Kids really pick up on the practicality of the rope and understand 90 degree angles when seeing how they are used in real life.
Review a written explanation of the theorum here.
If you have older students (7th grade and up) you may wish to use this online lesson plan for a more in-depth study of Pythagoras.
You may want to watch a few kid-friendly videos explaining the theorum.
Now make your own rope and try it out for yourself!
The fun thing about this lesson is relearning these concepts in a very practical way along with the kids. It's also a great opportunity to introduce kids to architecture.
I'd love to hear how this assignment worked out for you!