Why didn't they tell him in the beginning about those other things.

All they cared about was showing their new member off to the rest of the troop, and you see how it ended.
First I thought I'd go in and talk to Skinny and tell him he was a wonder, for that was just what he was, and Mr.Ellsworth knew it, too.
Then I decided that I'd better not on account of Connie.

And anyway, I wouldn't have any right to go in and spoil what our scoutmaster said, would I?
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