In the book Holes by Louis Sachar, who stole Mr. Sir's sunflower seeds?

Mr. Sir gave up smoking and replaced it with eating sunflower seeds.  He carries his bag around with him daily, snacking on sunflower seeds.  

Magnet steals Mr. Sir's sunflower seeds one day.  Magnet offers them to the others in their group.  Everyone wants some except for Stanley.  Magnet tries to pressure Stanley into taking some seeds, and tosses the pack.  Stanley does not catch it, and the seeds spill inside his hole.  Worried about the seeds being found in his hole, Stanley attempts to cover them.  Mr. Sir arrives, determined to find out who took his seeds.  Some of the seeds in Stanley's hole are still visible despite his attempts to hide them.  Stanley then takes the blame for what Magnet actually did.  Even though Stanley pretends to be the thief, Mr. Sir does not think he was the one who took the seeds.