"What's $ 3 x?"

So Eliz, my 5-year-old, woke up from her nap and was eating a post-nap snack while I'm chilling here on the computer. I'm half-paying attention to her when she suddenly asks something that sounds like "What's $ 3 x?" (Just in case some perverts trawl around looking for pages on the internet with that three-letter-word I'm changing some of the letters. I hope it keeps the pervs out. Anyways...) I'm shocked that she's even heard the word because I'd like to think that I've done a fairly decent job of sheltering her from anything that's more complicated than what I'd like to explain. So I asked her to say it again, and there it was, "What's $ 3 x?" I'm still confused so I ask her where she learned it, and she says it's in her book. Now I'm thoroughly baffled because I am *pretty sure* that she doesn't have any books with $ 3 x u @ 1 content. Finally after lots of asking, she tells me she got it in her "Sweet Success" book. So I go and dig it out of her room and ask her to find the word. I'm hoping I'll find some context, INNOCENT context, with which I can explain this word.

So there it was... "The sacks were half empty."

I almost had a heart attack for nothing!!!