The puzzle piece does not fit. The shoe will not go on. Your child pushes it away and announces, with total conviction, that they can't.
They said it before really trying. That part is maddening, and it is also the clue.
What "I can't" usually means
Occasionally it is literal. More often it is one of three other things wearing the same words.
"This is harder than I expected and I do not like the feeling." Frustration arrives faster than skill does, and a small child has few ways to describe it. "I can't" is the shortest available sentence for a stomach full of something unpleasant.
"I would rather stop than be bad at this in front of you." By four or five, children have worked out that some things are watched. Quitting early is a way of controlling how the failure looks.
"I want you to do it." Sometimes it really is that. Which is fine, sometimes.
The response that helps depends on which one you are hearing, and the only way to find out is to slow down and look at what happened just before.
Why "you can do it" often does nothing
It is the natural reply and it tends to bounce off, because it argues with a claim the child was not really making. They are not asking for your assessment of their ability. They are telling you a feeling.
Worse, cheerful confidence can raise the stakes. If you are sure they can do it, then failing means letting you down as well as failing.
Praising the outcome has a similar problem. A child told they are clever every time something works has good reason to avoid anything that might not.
What tends to work
Say the feeling back first. "That one is tricky. It keeps slipping." You have not solved anything, and that is deliberate. Being understood settles a child faster than being encouraged.
Shrink the ask. Not the whole shoe. Just the heel. Not the tower. Just one more block. Momentum matters more than the finished thing, and a child who is moving again usually keeps going without being asked.
Offer one concrete tip, not a pep talk. "Try holding it flat." Specific instructions are useful. Encouragement is just noise once frustration has arrived.
Name the trying, not the result. "You've had four goes at that." It sounds almost too plain to matter. It is the single most useful thing you can say, because it points at the part they control.
The long version
What you are building is not confidence exactly. It is a working theory that hard things get easier with repetition, which is a genuinely difficult idea when you are five and everything is new.
It gets built one unremarkable moment at a time, and mostly out of your reaction rather than your advice. A child who watches you treat a failed attempt as ordinary learns that failed attempts are ordinary.
That is not only a nice idea. In one study, toddlers who watched an adult struggle with a task before succeeding then tried harder at their own difficult task than toddlers who had watched the adult succeed easily (Leonard, Lee and Schulz, 2017). In another, children praised for effort rather than ability as toddlers showed a more resilient approach to challenge five years later (Gunderson and colleagues, 2013). What you model, and what you notice out loud, are doing more of the work than the advice you give.
The day they say "I'll try one more time" without being prompted, you will probably miss it. Say something anyway.
The Day [Name] Was Brave Enough is a personalized hardcover about trying the thing that feels too big, with your child as the one who does it. See the book.