Minecraft wiki entry for Water, associated text: A sponge instantly absorbs nearby water when it is placed next to water or when water comes into contact with it (by being placed next to the sponge, or by flowing toward it). A sponge absorbs water around itself (water source blocks or flowing water) out to a taxicab distance of 7 in all directions (including up and down), but won't absorb more than 65 blocks of water (water closest to the sponge is absorbed first). The absorption propagates only from water to water and does not "jump over" non-water blocks (including air).