To make a fluffy sponge cake, whip the egg mixture for at least 10 whole minutes. The eggs should be very pale in colour and tripled in volume. You want to incorporate lots of air into the batter so that your cakes will be fluffy once baked. Gently fold the flour into the eggs and don't open the oven door while the cake is baking until it is ready to come out to prevent it from deflating.