What are imaginary numbers good for?

I like doing math with my son Nicholas, who is about to enter the 7th grade. Yesterday, he asked me the question in the title of this post, and it stumped me. I know there are some applications in physics for imaginary and complex numbers, but they are too advanced for a 7th grader (or, I must admit, for a 49 year old applied macroeconomist). Off the top of my head, I could not think of any applications within basic economics or to everyday life. Can some commenter help me out?