Why does your sponge sink in the middle?

A sponge sinks in the middle primarily due to its structure and the distribution of air pockets within it. As the sponge absorbs water, the denser water fills the pores, causing it to become heavier and less buoyant. Additionally, if the sponge is not uniformly saturated, the uneven weight distribution can lead to it sinking in certain areas, particularly the middle.