Final answer:
The passage does not explicitly state what Johnny is shaking about, but the narrator suspects it might be related to a paper that both Johnny and Jennie have been secretly looking at.
Step-by-step explanation:
Based on the given information, it is not clear what Johnny is shaking about as the passage does not explicitly mention it. However, the narrator suspects that it might be related to the paper. The narrator has observed Johnny and Jennie looking at the paper secretly, which leads them to formulate a scientific hypothesis about the paper's potential influence on Johnny's behavior.