I recently received an email from someone who had read my old 
<a href="http://mattgemmell.com/articles/programming/">Introduction to Programming article</a>. 
He asked a very interesting question, and I'm soliciting suggestions as to how to answer it.

I have read your article and I must say that programming sounds to be pretty fascinating. I love the idea of creating something and then applying it to perform the action you created it for. I’m just wondering though how do you know that the code you put in to write the application is going to perform the task you intend? I know you are probably thinking this is a stupid question but I do not have any computer knowledge but am very eager to learn and comprehend it. So if you could please explain that to me I would greatly appreciate it. I thank you for your time and for the knowledge you have already given me to soothe my baffled mind. I hope you have a great day and thanks once again.

The trouble is, my polite friend is wrong about it being a stupid question; in fact, I'm having 
trouble coming up with a truly satisfactory and accurate answer which won't be discouraging. 
Any suggestions?