Many cooks will say to you that cooking perfectly boiled egg may be the hardest thing ever. Once the bile is too dry, sometimes the albumen or the yolk is not cooked properly.
This is because the egg needs two different temperatures because the yolk and the white are two entirely different structures. The white of the egg needs temperature of 180 degrees in order to be cooked perfectly, while the yolk needs 170 degrees.
A chef realized how to perfectly boil an egg. He put several eggs in pre-boiled water for half a minutes and added several ice cubes in order to reduce the heat. Then, every second minute he drew out one by one egg from the water. This is how he found out how many minutes he needs to boil the egg perfectly.