What's that, Jack?

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Jack and George are resting quietly when BOOM! A huge and strange ball lands beside them. "What's that, Jack?" "I don't know, George. Maybe it's a rock?" No--too soft. But it rolls. Fast! Jack, George and the ball roll right off the cliff and now it's a parachute. But watch out, they're going to land in the river... Jack and George have a brilliant day full of adventure with this object that changes with the landscape, then turns into a blanket to keep them warm.