I usually work on the assumption that the customer is stupid, but as he's the one paying my wages, then I have to do it very politely. I talk them through what I would do - "First check the power, has the plug accidently been removed, has a fuse blown, tell me what happens when you switch the printer off & on" - slightly better than asking is it plugged in & switched on. Similar techniques for making sure he's got paper & ink.

Only experience can really give you an idea of how dim-witted people can be. Like the operator who stuck her fingers into a moving fan when told to clean the air-filter, the safety guard told her to turn the fan off before removing the guard, but as it didn't say so in the instructions she'd been given, she thought it wasn't necessary.

I always aim to write applications with that level of stupidty in mind, it may seem completely obvious to me what needs to be done, but there's always the one user who can catch you out. (Back to the client who started testing by leaning on the keyboard - he would complain bitterly about messages such as "Press any key to continue" - he would deliberately press Ctrl or Alt, his view was if it says "Any" key, then it must be any key, otherwise after 3 presses the user would reboot the machine, as it's obviously not working - All occurences were replaced with "Press space bar to continue" & modified so that only spacebar worked)