Puzzles/Geometric Puzzles/Rubber Band/Solution 2
1. By stretching the rubber band, while holding the loose end to the nail, you can measure 4 cm. Draw a circle with radius 4cm around the nail.
2. Unstretch the rubber band and, while holding the loose end to the nail, draw a circle with radius 3 cm around the nail.
3. Stretch the rubber band. (It's 8 cm long now.) At the point, where it crosses the 3cm-line, fix it with your left-hand finger. The rest of the rubber band is 5 cm long. Turn the 5cm-part, so that the free end lies on the 4cm-line from step 1. Put your right-hand finger there.
The triangle "left finger" - "nail" - "right finger" has lengths 3, 4 and 5. Therefore it is a right-angled triangle.