baby stones
The dried-out, hardened remains of sperm that collects, along with hair and dirt, at the bottom of the bath drain. Depending on how many males in your household that partake in masturbation while bathing, enough "stones" can be built up to prevent adequate drainage in as little as one week. Baby stones can also be found in sink drains.
"Today I had to clean out the bath drain, there were enough baby stones in there to choke a donkey."