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@leighalexander Chocky (original John Wyndham novel is for adults, but there was a TV adaptation for kids). Boy hears a voice in his head, it's an alien telepathically communicating with him to help steer humanity's technological progress, hijinks ensue.
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@leighalexander Woof! and Bernard's Watch are not quite right, but close (Woof! : boy turns into a dog, has to hide that with the help of his friend, Bernard's Watch: boy gets watch that can stop time)