Group the sheets in 4s. Smear each with butter and put them on top of one another. On the fourth, put feta cheese on the short side and roll them up together.
Cut them into slices 3/4″ (2 cm) thick, place them in a greased baking dish (a small 9 3/4″ (25 cm) one) with the cut facing part up. Beat the eggs with the sparkling water and pour this over the phyllo pastries.
Put it in a cold oven. Bake at 428°F (220 °C) until golden.