Put the egg yolks in the mortar. Break them with the pestle and stir, blending them together. Add the oil, drip by drip, stirring all the time. When you have added half the oil, and the mixture is very thick and sticky, thin with a little lemon juice. When you have a perfect emulsion, add the oil in a slow flow rather than in drips.
You may not use all the oil, as egg yolks vary in size