The basic idea: a question without an explicit counterpart in the first clause
In sluicing with an explicit correlate, someone, something or somewhere already names an indefinite participant or circumstance. In sprouting, no corresponding element is expressed. The wh-expression asks about a parameter understood from the event. This may be a verb argument or an adjunct.
Compare He’s meeting someone, but I don’t know who, where someone is explicit and who refers back to it, with He’s writing, but I don’t know what. Nothing in he’s writing names what is being written. What “sprouts” from the fact that write normally permits an object.
The term sprouting describes the absence of an explicit correlate for the relevant wh-phrase. A full paraphrase helps check the intended question. You do not need to adopt a particular theory of unspoken structure to use the pattern.
event without an explicit counterpart for the missing detail + know / say / tell … + wh-phrase
He's writing, but I don't know what.
He's writing, but I don't know what.
What has no expressed correlate in he’s writing. The object is fully implicit and recoverable from the meaning of write.
She donated generously last year, though she won't say how much.
She donated generously last year, though she won't say how much.
The first clause contains no phrase naming an amount. How much is recoverable because donating involves an amount.
2. Check what the question is asking
An object question with what needs an action that permits that kind of object, such as write, eat or design. The first clause must also sound natural without its object. A dictionary label such as transitive does not guarantee every possible shortened construction.
Sleep has no ordinary direct object comparable to a written text. Without extra context, He was sleeping…, but I don’t know what therefore does not create the intended object question. However, why, where, how long or how much can relate to sleep. Intransitivity does not ban sprouting altogether.
Check the full paraphrase together with the context. Changing the verb may change the available question type, but a transitivity label alone is not an automatic test.
He was writing all morning, but I have no idea what.
He was writing all morning, but I have no idea what.
What asks about the written content. Writing appears without an object, and the action’s meaning makes the missing detail recoverable.
The committee is reviewing, but nobody will say what.
The committee is reviewing, but nobody will say what.
Review permits an object, review something. No object is expressed in the first clause, allowing the shortened what question.
Types of sprouting: object, recipient, source and amount
Different kinds of arguments can be recovered according to the verb: a direct object with what, a recipient or destination with to whom, a source with from whom, or an amount or degree with how much. The intended role determines the wh-phrase’s form.
When there is no explicit prepositional correlate, retaining the preposition is especially important: write a letter … to whom; borrow money … from whom. The possibility of leaving a preposition at the end of a full question does not itself permit dropping it from a short sprouting question.
In some short conversational questions, the preposition can follow the wh-word, as in who with, but it is still spoken. Do not confuse that reordering with leaving the preposition inside the omitted clause. Clear, reliable patterns here include to whom, from whom and for how much.
He wrote a letter, but I don't know to whom.
He wrote a letter, but I don't know to whom.
The direct object a letter is explicit. The recipient is not named and is recovered through to whom.
She borrowed money, but nobody knows from whom.
She borrowed money, but nobody knows from whom.
Borrow involves a source. From whom asks about it even though no source phrase appears in the first clause.
4. One component is explicit; another is unexpressed
A sentence can name the item while leaving its recipient, source or price unexpressed. A letter, the car or money does not block a question about another missing detail. These details are not all analysed as two direct objects of the verb.
The explicit argument, such as a letter or money, is not itself the correlate for a wh-remnant referring to a different role. To whom does not refer to a letter. These are distinct argument positions, not the same one shortened a second time.
That is the contrast with sluicing that has an explicit correlate: there the wh-word corresponds to visible wording; here it corresponds to an unexpressed role associated with the same event.
She sold the car, but I don't know for how much.
She sold the car, but I don't know for how much.
The car explicitly names the sold item. Its price is unexpressed and is queried through for how much.
He owes money, but I don't know to whom.
He owes money, but I don't know to whom.
Money identifies what is owed, but not an exact amount. To whom asks about the unnamed creditor.
Sprouting and ordinary sluicing: the same remnant, different conditions
In the simplest comparison, explicit something provides a correlate for what. Without it, the meaning of what is recovered from the event and context. Both cases involve sluicing; sprouting names the subtype without an explicit correlate.
The surface form of the shortened question can be identical. The distinction lies in the presence or absence of an explicit counterpart in the first clause. Understanding that boundary is an advanced C2 skill rather than merely memorising the form.
She's buying something, but I don't know what.
She's buying something, but I don't know what.
Sluicing with an explicit correlate: what corresponds directly to the already stated indefinite something.
She's buying, but I don't know what.
She’s buying, but I don’t know what. The first clause does not explicitly state an object.
Sprouting: the same remnant what now has no explicit correlate. The object of buy is entirely implicit.
Boundaries: arguments and adjuncts
Adjunct questions with why, when and how can ask about an event’s reason, time or manner without an explicit correlate. These can also count as sprouting. Object what requires a different check: what thing or content does the particular verb permit?
Some apparently intransitive uses still retain an implicit argument. Eat or drink without an object still implies food or drink, so what can be recovered after them too.
With sleep, how much can refer to the amount of sleep: He slept, but I don’t know how much. How long asks more clearly about duration. The lack of an ordinary direct-object what question does not rule out every question about amount or time.
They were eating, but I couldn't tell what.
They were eating, but I couldn't tell what.
Eat can appear without an object while still implying food. The implicit object can be queried with what.
He collapsed, but the report doesn't explain why.
He collapsed, but the report doesn't explain why.
Why asks about a reason, which can be associated with an event such as intransitive collapse. It does not test whether the verb takes a direct object.
Using sprouting: register and naturalness
Sprouting is familiar and natural in everyday speech and writing, especially with verbs of communication, creation and exchange such as write, cook, pay, donate and borrow, when a detail is unimportant or unknown to the speaker.
Excessive reduction where a verb genuinely does not license the intended argument sounds broken rather than economical. C2 control involves knowing which relationships the verb permits.
A full version with the argument expressed, such as I don’t know what he was writing, remains available and may be clearer. Sprouting is a stylistic option based on real structural conditions, not an obligatory shortening.
The lab is testing, but the memo doesn't say what.
The lab is testing, but the memo doesn't say what.
A natural, economical formulation. The fuller what the lab is testing is also correct and may sound more explicit or formal.
I know she's been designing, but she won't tell anyone what — not even her business partner.
I know she's been designing, but she won't tell anyone what — not even her business partner.
Sprouting remains natural inside a longer sentence. The added material does not require the object to be named in the first clause.
Common mistakes
Incorrect:He was sleeping all morning, but I have no idea what.
Correct:He was writing all morning, but I have no idea what.
Without additional context, what does not identify an ordinary object of sleep. The writing version describes a different event with an available object. It demonstrates a contrast rather than preserving the original fact about sleeping.
Incorrect:He arrived, but nobody knows what.
Correct:He arrived, but nobody knows when.
In its ordinary meaning, arrive supplies no direct object for what. When creates a clear question about arrival time. This can also be sprouting if no explicit time correlate is present.
Incorrect:He was designing, but nobody knows what was he designing.
Correct:He was designing, but nobody knows what.
After knows, the full embedded question is what he was designing, without inversion. It can be shortened to what. The correctly formed full version is also acceptable. Sluicing retains the wh-phrase rather than deleting it with the rest of the clause.
Incorrect:He sold the car, but I don't know for how many.
Correct:He sold the car, but I don't know for how much.
The price is an amount of money, treated as uncountable, so use how much rather than how many.
Incorrect:She borrowed money, but nobody knows of whom.
Correct:She borrowed money, but nobody knows from whom.
Borrow introduces the source with from, not of. The sprouted prepositional phrase retains the preposition appropriate to the verb.