The site has been, so it seems to me from the outside, undergoing a lot of changes since I started playing. Sometimes the site has worked differently than other times. For example, there was a period where the XP ('Energy' as it is now labeled in English) was not being decremented when you tried a bad solution. At one point you could regenerate the XP by going to a different challenge. I don't know if that still works. At another point I was convinced that the XP regenerated about every hour.
Basically, I think the XP is a brute forcing mechanism as noother said but I don't find it much of an annoyance. I've never gotten close to using it up, and as you solve more challenges you get more XP. Right now, I've 46 XP. I could get wrong answers all day, unless of course I were actually trying to cheat by brute forcing a solution.
Other than the server occasionally being slow, and that could be because I am half a planet away, I don't have any real complaints. I wish the 'forum' such as it is, were better though.