Why’s the Sea So Salty? Because You City Folk Pee Whenever You Go Swimming!

Gintama.: Slip Arc · Episode 19 · “Why’s the Sea So Salty? Because You City Folk Pee Whenever You Go Swimming!”

Consists of episodes animated from unused manga chapters, before returning to the Silver Soul Arc on January 8, 2018. Porori is an Onomatopoeia for something (mainly water) falling.