Rinse a cup of lentils and bring them and cook them for about half hour. Drain the lentils and let them sit.
Wash the potatoes thoroughly, and boiled them with their skin till they are tender. Drain them and halve.
Trim tender celery stalks and cut them in long strip or in circles.
Put the lentils, the potatoes and the celery on a serving dish and toss with olive oil and vinegar. Season the salad with black pepper and salt.