As an adult I've discovered the joys of RPN for calculators. I use free42/plus42 on my phone which is an opensource simulator of the HP42. My kids havent started using calculators yet, but I'm debating whether it's a good idea to teach them RPN. I think it's a better way of using calculators, but don't want them to get confused in class. Add to that there isn't a simple RPN calculator that I know of.