The "challenges" at [snip] are not that good, the site just posts obfuscated code and bad C grammar and asks if you can work out what it does. Much better are the challenges at; these actually help you learn how to use your tools (the compiler) to solve well designed problems as well as giving you some motivation to complete further challenges.

If you want to have a go at figuring out what obfuscated code does the best place for that is the IOCCC - the International Obfuscated C Code Competition, where all the winning entries from previous years are posted. My favourite is still the one that starts with main(){} then #includes itself 6 lines later (and works!), and there's a brilliant one where the code as is is a sort program, then you compile it and run it against its own source code whereupon it becomes a different program that not only works but actually does something useful (reverses the lines or something), then do it again and it becomes a 3rd useful program. Now THAT my friend is impressive.