By design, a sponge absorbs water and expands. Unless a sponge is so small that it is able to easily slip through the drain, it lodges within pipes if flushed down a toilet. As a result, the toilet either flushes sluggishly with less power, or clogs and possibly overflows.
What Likely Happens
Most toilet trapways are between 1 1/2 and 2 1/2 inches in diameter. The trapway is the curved pathway that serves as the route for waste and water to exit the toilet. If the sponge is a cleaning sponge, which is normally wider and longer than 2 inches, the sponge is likely to become lodged within the trapway. This clog prevents other materials such as waste and water from freely flushing out of the toilet.
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What to Do First
After a sponge is flushed down a toilet, do not flush the toilet again. This is often a natural reaction to see if the sponge flushes down the drain. This only raises the bowl's water level and possibly causes the bowl to overflow. Instead, turn off the water supply to the toilet by twisting the shut-off valve located behind the toilet completely to the right.
What to Try Next
If the sponge has not traveled too far down the toilet drain, a plunge is sometimes effective. Plungers are designed to create a suction effect that works on shallow clogs. If the sponge is lodged in the trapway, use a closet auger, also called a snake, to pull it back out. It is sometimes necessary to remove the toilet from the flange and access the toilet trapway from the bottom of the toilet. Remove the sponge with a gloved hand or the closet auger from underneath the toilet. Be sure to remove all the water from the toilet tank and bowl first, and install a new wax ring before you place the toilet back onto the flange.
Prevention
Always throw a sponge away in the trash instead of flushing it down the toilet. Outside of being cautious when cleaning the toilet with a sponge, the best practice is to always keep the toilet seat lid down when the toilet is not being used. This is also advised when small kids are in the home to prevent them from dropping foreign objects into the toilet.