Well, it wasn't yet boiling and I was holding him and could pull him away, so I let him gingerly put a finger into the water. He cried, we washed his finger in cold water, and a few minutes later all was well. Since then I've never, ever had to worry about him near anything hot, even as a toddler. And more generally when I tell him something is dangerous he internalizes it well--doesn't mean he won't refrain from something, though. Maybe the carefulness is more likely a coincidence or his nature, but the lesson about hot things burned into his consciousness.

It's obvious in retrospect, but I guess a parents job isn't to prevent their kid from hurting themselves, it's to make sure that when they hurt themselves only the lesson is lasting. More recently it's become evident to me that not only do they need to hurt themselves to learn, but it's important to make it a positive experience, the pain notwithstanding. For example, now whenever toddler #2 falls down on the pavement my refrain is "good fall!", and I imitate dusting my hands off. I'll keep my distance unless it looks like she needs help getting up or consoling, but try to do it calmly and positively. Now, a couple of months into it, whenever she falls she jumps up, smiles, and brushes her hands, even if she's scraped. I did something like this with her brother but never so deliberately and consistently.

Somewhat relatedly: I think it depends on the kid and the family, but neither of my kids respond well to loud voices, no matter if well meaning or urgent. It was frustrating for me to change my habits because when I was growing up elevated voices weren't negative, just a signal to pay attention. Maybe it's genetic or maybe just how their mother has habituated them, but if I elevate my voice they invariably take it negatively. They seem to pay attention better and react more positively if I modulate my voice. Being conscientious about my voice also causes me to think twice about how I respond, even in imminent situations like you describe. Unless there's a risk of permanent brain injury or mutilation (at home, at least, usually self-evident), I try to tell myself to stop and slow down and consider whether I should just let nature take its course. It's worth the risk. That is, most of the time it's worth the risk to wait a second to consider the situation rather than just reacting, and if you do react it's then easier to do it calmly.

Not preaching, just sharing. It all definitely depends on the parent and the kid and the situation, but I've learned a lot from people sharing their parental experiences and anecdotes on HN, even when I disagreed or didn't find it applicable.

Those are of course situations where immediate action IS necessary. It's a judgement call, but the point seems to be that if it's not a mortal danger, then don't overreact.