I was eating a meal of chicken, peas and corn with a Japanese person. She could understand why it was correct to say "chicken" instead of "chickens", but asked me to explain why we say "peas" but not "corns". To her they look very similar - just one vegetable is green, and the other is yellow. I came up with a few attempts at an explanation, but never a satisfactory one. Can anyone help me?
Yes, the pea is an anomaly because of the history of the word. I think, in general, that whether we choose the singular or plural of a food depends on whether we are thinking of it as an object or as an uncountable substance. "Chicken" is usually singular but a bird that is small enough for a dish to contain several, served whole, could well be used in the plural - quails, for example.
Cereals tend to be used in the singular, I assume, because the individual grains are too small for us to think of them as singular objects. This thus applies to corn (maize) even though it has relatively big pieces - about the same as a pea, as it happens. I use the plural "corns" for baby corn cobs but I don't know if that's usual. (I'm not sure, BTW, that maize should really be called a cereal, although cornflakes are the archetypal breakfast "cereal".)
The individual pieces of corn are called "corn kernels" aren't they? They are not called "corns" in the plural, because the word "corn" does not refer to an individual piece. I would suggest the anomaly comes about because corn can be served both on the cob, where it doesn't make sense to think of it in terms of little pieces, and as a pile of kernels taken off the cob.