sentence: A man is holding a pocket knife while sitting on some rocks in the wilderness. sentence:  Then he takes a small stone from the flowing river and smashes it on another stone. sentence:  He starts to crush the small stone to smaller pieces. sentence:  He grind it hard to make the pieces smaller. sentence:  Then he takes a piece of bark and rubs the powered stone pieces onto it. sentence:  The stone particles stick to the wet piece of wood. sentence:  He then takes the knife and sharpens it against the wood piece. sentence:  He then notices a little black and green poisonous frog sitting next to him. sentence:  The frog then escapes and jumps away. sentence:  He explains how the frog secretes a poisonous fluid that can be extremely harmful.