The nine-year old is learning about cross-section diagrams in school. One of the examples in his book was the cross-section of a grape. He was running through the diagram -- skin here, stalk here, seeds here -- when the six-year chanced in on us.
"Grapes don't have seeds," she corrected him.
I had intervene and cut off the impending argument.
"All fruits have seeds," I told them, "It's just that the grapes we buy have tiny, tiny seeds so you guys don't have to spit them out when you eat them. Just like all watermelons have seeds, but we buy the ones with small seeds."
In case you thought those stories of children thinking milk came in cans was apocryphal, there is at least one smart six year old who thinks "seedless" grapes are the norm.
p.s. I looked this up later -- "seedless" varieties vary in how small the vestigial seed traces are and in how hard the outer covering of these traces is. Apparently, even the same variety can have different sized seeds in different years.