Johnny and the Bomb

Johnny Maxwell finds old Mrs. Tachyon, a bag lady, in the cinema, and he learns that her grocery cart is, in fact, a time machine. Johnny and his friends go back in time to 1941 when his town was accidentally struck with German bombs and many people were killed. One of Johnny's friends is left behind in 1941, and they make plans to return for him.

Stories about time travel are always fun, I think, and this one is not only a story about time travel but it's also written by the masterfully funny Terry Pratchett.

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