Short Answer: Magnet stole Mr. Sir's bag of sunflower seeds.
In chapter 19, Mr. Sir drives out to the location where the boys are digging their holes and gives them a routine water break. After filling their canteens, Mr. Sir drives away, and Magnet surprisingly offers the boys some sunflower seeds. While Mr. Sir was occupied with filling the boys' canteens, Magnet stole his bag of sunflower seeds.
The boys proceed to toss the bag full of seeds around and indulge in the rare treat. Zigzag then throws the bag of seeds at Stanley, who accidentally drops the open bag in his hole. The seeds end up spilling in Stanley's hole, and he immediately attempts to sweep them back into the bag. Suddenly, Zigzag calls out to Stanley that Mr. Sir's truck is returning, and Stanley desperately begins to shovel dirt over the seeds and bag. Unfortunately, Mr. Sir discovers the bag of seeds in Stanley's hole and takes him to see the Warden about the incident.