What it teaches
Sorting things into living, once living, and never alive. The middle group is the whole point. A wooden table, a woollen jumper, a leather shoe and a piece of paper were all alive once, and a child who has never been asked about them assumes they belong with the rocks.
Underneath is the idea that life is a property of matter rather than a kind of matter — a long way from being said in those words, but it starts here.
What to look for
The arguments are the good part. Is a seed alive? Is an egg? Both are, and both surprise children. A disagreement here is worth more than a right answer.