"Some furniture is made of wood. Some wooden things are trees. Therefore some furniture is a"…

Why

Nisha's two starting sentences ('some furniture is wooden', 'some wooden things are trees') are each individually true, which makes the conclusion feel earned — but the SAME 'some A are B, some B are C' pattern can be filled in with true starting sentences and produce an obviously absurd conclusion, which is exactly what happened here: furniture is never literally a tree. The reason this pattern fails is that the word 'some' in the first sentence and the word 'some' in the second sentence might be pointing at completely different pieces of the middle group — 'some wood' that is furniture and 'some wood' that is trees do not have to be the SAME wood at all. Test it: a wooden chair (furniture made of wood) and a growing oak (a wooden thing that is a tree) can be totally unrelated pieces of wood — nothing forces any overlap between them, even though both starting sentences are true. Her friend is right: 'some A are B, some B are C' never reliably proves 'some A are C' — this reasoning pattern breaks even when every individual sentence in it is true.