Why does toilet paper break apart in water?
Short answer
Toilet paper is designed so that water can enter its structure, weaken the bonds between its fibers, and let water movement pull those fibers apart.
How it works
Paper is made from plant-based cellulose fibers that are held together in a network. When the paper is dry, bonds between the fibers help it keep its shape. Water makes the fibers swell and slide more easily, so the network loses strength.
Toilet tissue is made to balance two needs: it should hold together during use but separate relatively quickly after flushing. Kitchen towels and some wipes are made to stay intact when wet, so they do not break apart in the same way.
Key point
Paper that breaks apart in water can still cause a blockage if too much is flushed at once. Follow local guidance and flush only toilet paper and human waste unless your plumbing provider says otherwise.