Grasp the egg between thumb and fingertips and carefully hit the egg's equator with an angle of about 45° against the rim of a pot, bowl or cup; repeat until a crack appears in the shell. Depending on the length of the crack, rotate the egg slightly and repeat process until a good half of the circumference of the egg is cracked. Then grasp the egg with both hands so that both thumbs are at the crack and the fingers are at the two poles. Grasp the shell well with the thumbs and push your thumbnails or whole thumbs into the crack, while holding the egg over a pot, bowl or cup so that one pole is facing down and the other pole is facing up. Gently pull both thumbs away from each other. As soon as the shell breaks open, egg white will flow down into the bowl, leaving the yolk and some residual egg white in one half of the shell. By transferring the yolk from one half of the shell to the other, the rest of the egg white flows down into the bowl; the yolk can then be used separately or mixed with the egg white, as required by your recipe.