How do you answer THE QUESTION

When people ask why you don’t have children, what do you say? My favorite answer so far is “because I’ve seen yours.” Another good one is, “No, would you like to give me yours?” But really, a lot parents start their conversations with “Do you have children?” And when you say “no,” they want to know why. Depending on my mood, I just say, “No I don’t. God had other plans,” or I explain that I have three stepchildren but no kids of my own. Several of my friends just say they have dogs.
Do people ask you? What do you say?