Monster Blood II (Goosebumps #18‪)‬

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Evan Ross can't stop thinking about Monster Blood and what happened last summer. It was so horrible. So terrifying. Too bad Evan's science teacher doesn't believe him. Now he's stuck cleaning out the hamster's cage as punishment for making up stories.

Then Evan's friend Andy comes to town, and things go from bad to worse. Because Andy's got a present for Evan. It's green and slimy and it's starting to grow...