I've been learning lots about orbital mechanics and rocket science thanks to Kerbal Space Program. What a sense of accomplishment with each step

* made it to orbit

* flew by the moon

* crash-landed on the moon

* landed on the moon and returned with the stranded astronauts

* landed on Mars (stuck there for now sending back science experiment data)

* flew by Venus + Mercury

* Took a sample of two different asteroids

* Shot a little refueling space station into orbit and successfully docked to it with other ships