You know what they're good at. What they're capable of is still an open question.

What is the person you manage most closely capable of? Not what they're good at - you know that. Capable of is a different question, and if you found yourself guessing, you're in good company. Almost every leader is carrying an estimate they've never tested.

Worth turning the same question on yourself, too. You could list what you're good at without pausing. What you're capable of has been an open question for a while.

Capability stays invisible until something asks for it, which means the only way anyone finds out where their limits sit is by going somewhere near them.

Watching someone handle work they've done a hundred times only tells you how they handle work they've done a hundred times. Useful information, and a much smaller question than the one you're asking.

Dr Alia Bojilova made this point at the Revved summit last month. She's an organisational psychologist who spent years as the lead selection psychologist for the New Zealand SAS, watching who holds up under sustained pressure against a bar considerably more demanding than a performance review. Her claim is that capacity shows itself in challenge, and where nothing much asks for it, it quietly fades.

Your physio would recognise the everyday version: use it or lose it. A capability that never gets called on doesn't sit there fully formed, waiting for the day you need it. It softens, and so does the person's own sense of what they could take on.

This is also the main finding of the longest-running research on how managers develop. The Center for Creative Leadership has spent more than three decades asking accomplished managers where their most important learning came from. The consistent answer is challenging assignments, ahead of training and ahead of coaching.

Which is quietly reassuring, because the evidence you need is already in your own history. Nobody had to be extraordinary. Somebody just had to be given something difficult.

Stretch doesn't always work. Handled badly it produces someone smaller afterwards rather than larger, and that outcome is common enough that plenty of leaders have quietly stopped offering.

What separates the two has surprisingly little to do with the size of the thing.

Richard Lazarus and Susan Folkman spent decades studying how people size up a demand before they respond to it. Their account is that we run two quick calculations, mostly below conscious awareness: what's being asked of me here, and what have I got to meet it with? When the second looks roughly equal to the first, the demand registers as a challenge. When it doesn't, the same demand registers as a threat.

Same work, two readings, and a very different experience of the month that follows.

The useful part for a leader is that hardly any of this is fixed. Most of what sets that reading sits on your side of the table.

When you're assigning something big: say the outcome and stop. Then ask what they'd need from you to have a real go at it. The answer names whichever condition is missing.

When somebody says they're not ready: take it seriously without treating it as settled. "What would make it feel doable?" usually surfaces a resource gap rather than a capability gap, and those are yours to close.

When they're mid-way and struggling: ask which way it's reading. "Does this feel like something you can get your arms around, or does it feel like too much?" The answer tells you whether to add support or reduce scope.

When it's finished: ask what they did differently from how you'd have done it, and what that turned up. Then say out loud what you now know about them that you didn't before. That sentence is what turns the experience into a raised expectation for both of you.