Who stole the sunflower seeds in Holes?

In chapter 19, Mr. Sir leaves after giving the boys their routine water break, and Stanley returns to his hole to resume digging. Shortly after Mr. Sir's truck pulls away, Stanley hears Magnet ask if anyone would like some sunflower seeds. Magnet had stolen Mr. Sir's bag of sunflower seeds while he wasn't looking and proceeds to eat them one handful at a time. Magnet then tosses the bag of seeds to X-Ray and comments that he couldn't help stealing Mr. Sir's seeds because his fingers are like magnets. The bag is then passed to X-Ray, Armpit, Squid, and Zigzag, who ends up haphazardly throwing it to Stanley. Unfortunately, Zigzag never rolled the bag up before throwing it, and the seeds scatter throughout Stanley's hole. When Mr. Sir returns, Stanley takes the blame for stealing the seeds and is taken to see the Warden.