Complex sentence: Sponge cake is a cake based on flour (usually wheat flour), sugar, and eggs, sometimes leavened with baking powder which has a firm, yet well aerated structure, similar to a sea sponge.
Split into simpler sentences: Sponge cake is a cake based on flour (usually wheat flour), sugar, and eggs, and is sometimes leavened with baking powder. It has a firm, yet well aerated structure, similar to a sea sponge.