I was amused by the claim in the article about crop circles headlined 'Circled by mystery' (South China Morning Post, August 21) that 'you would need modern instruments to make such perfect circles'. Here is one simple way to make a perfect circle in the earth: tie a stick to one end of a rope and a marker to the other end. Plant the stick in the ground, and walk around it with the rope stretched taut. The path traced by the marker is a perfect circle. Many more-sophisticated geometric patterns may be created with only these tools, using the 'ruler-and-compass constructions' that were known to the ancient Greeks, and are still taught in some schools today. AARON HERTZMANN New York, US