Here's why:
* Stanley's shoes: Stanley is assigned to dig holes all day at Camp Green Lake. He's given a pair of very old and worn-out shoes that are too big for him.
* The "sploosh": The sound "sploosh" is made when Stanley's foot slips and slides around inside the oversized shoes. This happens frequently, especially when he's digging and his feet are wet with sweat.
The "sploosh" sound is a recurring element in the book, representing Stanley's discomfort and the poor conditions at Camp Green Lake.