How do you find the center of a circle with a pencil and a book? Given a circle on a paper, and a pencil and a book. Can you find the center of the circle with the pencil and the book?


Draw a right triangle inside the circle, making all vertices touch the circle.
Transform it into a rectangle.
Draw a line joining the opposite corners and you will have obtained the center of the circle
Edit:
The idea is very simple: 1. Put a corner of the book touching the circle from the inside in any place.  2. Use the edges of the book for drawing the catheti of the right triangle until they intersect the circle.  3. Draw the hypotenuse using those intersections.  4. Put a corner of the book in any of those intersections overlapping one edge with the cathetus.  5. Use the other edge of the book to draw a line until it intersects the circle.  6. Draw a line joining this new intersection with the intersection opposed to the hypotenuse (right angle point). 7. The point in which this line crosses (intersects) the hypotenuse is the center of the circle.  
Edit:

In fact, any rectangle inside the circle with every corner touching the circle will do the trick
