This is a pretty good definition of sarcasm, and it's impressive that five-year-old Jack not only understands what sarcasm is, but can pick up when someone is using it. That's more than a lot of adults are able to manage.

It's difficult to tell these two things apart. You can't really pretend without lying, and you can't really lie without pretending it's true. In Jack's mind, they're two distinct concepts. He doesn't understand that to an outside person, like Ma, they look like the same thing sometimes.

While this is technically true, this comment by Ma makes Jack unreasonably scared of germs. It also comes from a place of fear: Jack is her son and the only person Ma has, and she has to be really careful because she is not able to get him medical care.