ELI5 Decaf - how did a train accident lead to its discovery?

How is it possible to get the caffeine out of coffee beans? How was the process accidentally discovered? What percentage of caffeine needs to be removed for it to be considered decaf? Is it true that coffee makes you want to go to the toilet?

... we explain like I'm five

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: realnotfake, subzerot, another1mitesthedust, darthvirgin, gregw134, brackattack27, yalloc, gnonthgol and jwstap

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