So I've been playing quite a lot of Kerbal Space Program of late. It's a fun rocketry simulator/space sandbox game, but don't take the cartoony little green men to mean that it's an easy game. The science behind the game is deceptively robust and realistic, and even achieving a stable orbit of Kerbin (the local Earth analogue) takes some doing.
Anyhoo, the basic concept of the game is simple: you've got a star system to explore, a space centre to launch and coordinate your missions, and an endless supply of rocket parts with which to design your own fleet of spaceships to take an similarly endless supply of Kerbonauts to the heavens. Great fun for a space fan such as myself!
Lately, after building a fairly successful space station, I decided to try for a landing on the Mun, Kerbin's major natural satellite. Paying close attention to a helpful tutorial on the KSP wiki (and reloading previous saves quite a bit), I was finally able to land a Kerbonaut on the Mun's surface... only to find that I hadn't provided enough fuel for a return voyage. Whoops.