"Who" is to "whom" as "what" is to what?

"Whom" is used in place of "who" when asking whom an action was done to. For example:

To whom did he send the letter?

So this got me wondering if there is a similar word for "what." I did some searching, but didn’t find anything. I looked in multiple thesauruses for synonyms of "what" and "whom," but I didn’t find anything there either. And so I’m here asking, is there a similar word for "what?" If so, what is it?

For example, what word would go in the blank?:

____ did you break?