Πηγές: https://coolweb.gr/ti-einai-ouranio-tokso-pos-dimiourgeitai/
https://www.google.com/url?sa=i&url=https%3A%2F%2Fpt.slideshare.net%2Fssusere2ab45%2Fss-42162014%3FsmtNoRedir%3D1&psig=AOvVaw0NQB5l7-GW-Vcy9XR50ZXq&ust=1588168213468000&source=images&cd=vfe&ved=2ahUKEwjN8fbioYvpAhXWwYUKHSD7ApQQr4kDegUIARD4AQ
http://mariabos.blogspot.gr/2013/03/blog-post_5.html
http://taniamanesi-kourou.blogspot.gr/2012/10/blog-post_5406.html