What's the Name of That Book??? discussion

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Covered with Nails
SOLVED: Children's/YA
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SOLVED. Mushroom hunt - a mushroom that can mimic objects [s]
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Is this a children's or YA book? I can't exactly figure that out from where I went in to his bibliography, since all I did was look at the titles.
Oh cool. For shelving purposes, it's part of this collection...
Covered with Nails: and Other Stories to Shock Your Socks Off!
Covered with Nails: and Other Stories to Shock Your Socks Off!
There was something like a mushroom hunt or a scavenger hunt and there were two main teams competing (I'd say they were middle school aged). The prize (for the most mushrooms??) was a massive chocolate something? A team of bullies stole the other team's sack full of mushrooms and were going to win. But then someone in the first team had found a special mushroom and it could mimic things (a bin stands out).
In the end, the (view spoiler)[ mushroom turns into the chocolate prize and the bully wins and eats it and I seem to remember that it exploded or something (hide spoiler)].