What is the best way to teach a child to stay away from danger?

Inspired by this question (thank you, @Hypocrisy_Central), I just kept on feeling that it was very frustrating to see that question end up in a bitter quarrel between opposing views on corporal punishment where people felt (and were) personally attacked.

It left me feeling that we are all just wanting the best for our children and I would like to try it again, with a different approach. One we can hopefully learn from rather than be bitter about.

So (while borrowing from the other question):
”Lenny” [10ish] got caught in the tool shed attached to the back of the house, playing with matches again. He could have set off the paint, solvents and welding equipment and blown up half the house; putting himself, his mother and younger sister in danger.”

Just off that info, what is the best way to change Lenny’s behavior and be more ‘safe’?

Please feel free to elaborate on the topic though. I would also like your view on the approach to this topic in general. For olde as well as younger children.