The idea for this instructable came to me during the latest lecture about physics.
All credits goes to Pythagoras (572-497 BC).
It may not have any practical application unless post-apocalyptic survival, but it's great for making with kids to demonstrate them the most fundamental laws of Universe.
Step 1: Let's Do It!
All you really need is some twine. The best is thin one. Pins are helpful but if you have third hand not necessary.
Tie a knot somewhere at the beginning of the twine. Tie second knot about 10 cm (4 inches) far. Now you've got your basic distance. Measure it and use to tie next knot. Be careful not to measure them incrementally, because deviation also grows incrementally; instead of this use only first distance. Repeat until you have 12 knots. While tying 13th knot put the first one inside and fasten the way you end up with loop with 12 knots.
Grab first, third and seventh knot and stretch the loop out. You have perfect right triangle!
Step 2: How Is It Done?
Right triangle edges ratio is:
a² + b² = c²
It's discovered by Pythagoras. In our triangle edges are 3, 4 and 5 generic units and fits the equation:
3² + 4² = 5²
9 + 16 = 25
For most of you it's nothing new but it's always fun and pleasure to "see the math":)