The Marriage of Two Minds, Admitting Impediment

I feel a dolt when I've "corrected" Son's 2d grade homework (and usually I think I've gone over it rather well) and it comes back from school the next day with one of his answers wrong, and circled by the teacher. I guess a few get by me. Which gives extra pleasure when one comes home from school unnoticed by either teacher. It doesn't happen often, but today's is great fun.
Q: What is the meaning of "gal"?
A: Wife.
Which is lovely, but when the question is posed by a Math Facts sheet, I suspect Son meant "gallon," instead?