Number 29
The Bicycle
Every safe thing we have was made safe by failing first. Some failures do not let you.
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No second try
Trial and error is how everything else got made safe: fail, survive, study what happened, fix it. That method has two requirements which are easy to miss because they are almost always met. The mistake has to be one you live through, and there has to be time afterwards to look at it. Take either away and the method stops being available, and nothing else in the notebook replaces it.
In this storyThis is the one that does not get repaired. Kenji checks his own list first, and he is careful: nothing he would never trade, nothing to break, it is only a hill. The wish is granted exactly, and a brake is not part of being fastest. He has the correction ready and about two seconds to say it, going the wrong way down the hill. Mei writes the notebook page alone, and the title is in her handwriting because he is not there to write it.
Why we only get one try on safeagi.ca goes into it properly.