Meg is Eva's Kindergarten teacher. One morning, while another parent was dropping off his child, he was chatting with Meg and mentioned that he had been up all night looking after his neighbor's kids. The neighbor had been ill and had to be rushed to the hospital.
Meg, concerned, asked if the neighbor was all right. It turns out he was, but at the time it had been scary: he'd had a serious allergic reaction. Meg asked to what. The parent thought it was to celery, but thought (as did Meg) this was odd, and hadn't previously heard of a celery allergy.
At this point, Eva, who had been playing with K'Nex (and apparently ignoring this conversation), piped in: "Some people are allergic to celery, but only if they exercise right after they eat it."
This is pretty typical of Eva. She appears to be doing whatever her six-year-old thing is -- putting all the translucent LEGOs together, poking a dead ladybug to wake it up, writing menus for her latest restaurant -- but she is soaking up whatever the nearby grownup conversation is.
And indeed, that is how she happened to have this particular piece of obscure medical information. Becca had seen this very patient, and told me about it the next morning -- while Eva appeared to be more interested in her breakfast.