The Green Futures of Tycho — William Sleator

When eleven-year-old Tycho discovers that the mysterious egg-shaped object he dug up in his garden is a time travel device, he can't resist using his newfound power. Soon he is jumping back and forth in time, mostly to play tricks on his bossy older brothers and sister. But every time he uses the device, he notices that things are different when he gets back -- and the futures he visits are getting darker and scarier. Then Tycho comes face-to-face with the most terrible thing of all: his grown-up self. Can Tycho prevent the terrible future he sees from coming true?

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