You know when you sharpen a pencil, right? And you're excited, because you're going to draw a picture of your family outside your house, and you've learned how to draw bricks and what're probably seagulls.
But when you apply pencil to paper, the nib of your brilliant new sharp pencil gives, and flakes out to one side.
Pulling out the nubbin of graphite, you start to sharpen the pencil again. But now, you have about an eighth of an inch of futile non-sharpening, in which there is no "lead", just a broken collar of wood. While you do this, the mental image of your family outside your house is fading, like the photo of Marty McFly in Back to the Future.
After seconds seeming like minutes, you will have sharpened the pencil again, and paid no heed to the clicking sound that didn't feel like a natural part of the pencil-sharpening process. This recklessness will come back to haunt you, sooner than you think.
With your tongue hooked over your top lip, you start to draw the outline of your father's head. After a quarter of the circle is completed, your fingertips sense something awry, and it feels like a premonition when the pencil lead snaps once again.
That feeling of foresight leads to an overwhelmingly frustrating sense of I could have done something to stop it. This, heaped on top of the injustice of a twice-snapping pencil, can bring tears of impotence to the child who just wants to draw his mummy and daddy holding hands.
The explanation given by mothers and scientists in this situation is that "it's probably snapped in the middle".

A similarly horrifying experience was if the snapped-off pencil lead stayed stuck in the end of the sharpener, causing the blade to slide impotently over the wood, no matter how hard you turned the pencil. Brown pencil crayons were particularly prone to this snapping-off phenomenon, and were guaranteed to provoke tears of hot frustration when you were in the middle of colouring in a picture of a big, fat poo.