This is an advanced skill and is best used with care and within a strong and established relationship.
It can be useful to challenge someone’s strengths particularly if they are feeling underconfident.
"I’ve heard you say that you have already achieved x and y so I’m wondering what is getting in the way of you achieving z?"
You may also challenge shoulds, oughts and musts as well as can’t. It is often enough just to offer back the word.
Other kinds of challenging may include offering back discrepancies such as:
On the one hand you're telling me it's not possible to do x and yet you've also told me that by thinking creatively you've already achieved y and z.
On the one hand you're telling me it's not possible to delegate this work to others, and on the other hand you are also telling me that two of your team are asking for more interesting and challenging work.