Stab
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.
し
さ
None
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?