I'd probably point out the difference in a non-corrective way. Either just re-cast correctly ("I want rice!" "Oh, you want some pasta? Here's your pasta!"), or say something like, "Yeah, it's soft like rice, and we eat it like rice, but this stuff's called pasta! See, it's longer, and it's shape is different, and it tastes a little bit different." And then move on. It's really no big deal... it's a part of how their inexperienced brains deal with stuff. They can also over-specify. Like the word "crib" only applies to a crib that is the same color is theirs, or "chair" only applies to kitchen chairs, and then they over-generalize and call everything in the living room a "couch." Just model correct language and they get it eventually.Â
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Over-generalization of words is very common at that age. DS, who's much older (2 1/2) still calls all football players on TV "Packers." If a team is playing, he'll say, "What team are those purple Packers?" "What team are those orange Packers?" DH and I are Packer fans, and I guess it rubbed off. He KNOWS the correct term "football players" at this point, but it's one of those weird linguistic things that has stuck.