It is so hard to know how to respond to stuff like this! I'm faced with it a lot at work, where I help supervise the interns--at least there I have a good reason for double-checking what they're doing. If I'm not sure about saying something, I usually do a quick calculation on offending them because they already know vs. what happens if I say nothing and they don't
know. If it's something serious I mention it (probably I have a lower threshold here than I would at home, though, since almost everything they do could affect the work environment somehow).
It's very handy to use a construction like, "A previous intern we had didn't realize he should always X, and the thing exploded!" Or, "You know you should always X, right?" and if they say yes I say, "Awesome!" and go back to what I was doing (instead of hovering or quizzing them like I don't trust their answer).
Sometimes with a friend or family member I try to take any sting out by over-explaining and passing it off as a quirk of mine. "Did you remember to roll up the hoses before mowing?" "Yes!
" "Sorry, I was just thinking about what would happen if you ran over them and then we'd have to go back to the store for more and put them out and get rid of the old ones, and that would be really sad..." in a tone that says, "Yeah, I'm being ridiculous, I know."
Better safe than sorry, when dealing with anything that could catch fire, flood, or otherwise be permanently damaged. The consequences of not knowing how to use an ice cube tray? I would probably let that pass.