し
さ
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There is a head pinned to a tree by a knife. This person has been stabbed! They were stabbed so hard that they were pinned to the tree here.
You want to know who did the stabbing, so you look around to see if the stabber is still in the area. But all you see is a sheep (し). Sheep can't stab people, right? They have hooves and probably can't pick up knives... right?