I found this lovely geometrical riddle in Martin Gardner’s book “My Best Mathematical and Logic Puzzles”. The most lovely thing about it is that Gardner gives a time limit: one minute! So, now draw out your stopwatch and read the instructions below. When you are ready, press “SHOW ME” below and the drawing will be revealed. After that you will have only 1 minute (if you wanna beat me, you have 35 seconds)!
Instructions: Click on SHOW ME and start your stopwatch at the same time. You should be exposed to an image of a rectangle inscribed in a circle. The question to be answered is: can you accurately determine the length of the line segment AC? Once you know the answer, glance at your watch to determine how much time is spent.
It would be fun if post your time as a comment!