How do you say something like that?
“There’s a thing for which I don’t know what it is” “There’s a thing where I don’t know what it is” “There’s a thing that I don’t know what is”
or (the one which I hear people say a lot but sounds awkward:) “There’s a thing that/which I don’t know what it is”?
To be honest they all sound awkward to me to varying degrees
I’m struggling to think of a context where you’d say this where you couldn’t just say “I don’t know (about) that thing” or “there’s a thing I don’t know”.
That there is a thing is kind of implied.