Beat the eggs. Put the baking soda in the yoghurt cup and stir. It'll effervesce. Stir well.
Pour it into the eggs. Pour water into the now-empty yoghurt cup, then into the mixture. Crumble in the feta cheese finely.
All that's left is the flour. The mixture needs to become very thick. Leave it for 10 min. to rest.
Smear your waffle-maker with butter and pour in the mixture.