I really like maths, computer science and riddles. Naturally, I have been a big fan of projecteuler.net for a long time now. It is a website with hundreds of mathematical riddles of increasing difficulty. For every problem, you need a good combination of maths and programming, although some problems can be solved with pen and paper only. I have even created projekteuler.de for crowdsourcing German translations of the problems. During the development of the website, I wanted to keep the number of problems as well as their titles and content up-to-date in order to allow early translation.