In article ,
Prometheus wrote:

Further trivia: If you put 12 equally spaced knots or marks
in a circle of string and have three persons holding knot
#1, #5 and #8 respectively and pull all three sides taut, it
will make a 90 degree angle at knot #5.

Interesting, I never heard that one.

Guess what you get when you add 3, 4, and 5 ? grin

Euclid's method is to take any
point on a circle, and then draw line from that point to the points
where any diameter crosses the circle. The resulting angle will
always be 90 degrees.
Aut inveniam viam aut faciam