Why is this nonstandard Esperanto?
"Kio estas en la bovlo?" versus "Kioj estas en la bovlo?" when you're pretty sure you're asking about multiple items.
This also applies if you're wondering what variety of items are in the bowl, not just one group of what are considered the same thing.
Talking about the same bowl with multiple apples in it, "Kio estas en la bovlo?" can have the following correct answers, depending on interpretation:
"What are the contents of the bowl?"
--> "Pomoj estas en la bovlo." (There are multiple discrete objects that are apples that make up the contents of the bowl)
"What single thing is in the bowl?"
--> "Fasko da pomoj estas en la bovlo." (The apples as singular set that constitutes the bowl's contents)
"What kinds of things are in the bowl?"
--> "La pomo estas en la bovlo." (The apple, as opposed to some other kind of thing)
Perhaps a system with plural "kioj" removes the ambiguity here, though I'm not sure! Maybe someone can explain why Esperanto doesn't have it. The other question here is: should I use it? I'm sure "kioj" would be understood, even if it doesn't appear in the Plena Ilustrita Vortaro de Esperanto.
I've found this article which explains this with "because English" and goes on to describe how English does it, but incorrectly. But that's all I've found so far on this topic.
EDIT: Now I'm also wondering about ĉiu vs ĉiuj