I've been using this site to get my basic foothold in Mandarin. If you scroll down you'll see a section entitled "WHAT", including some questions like "What's the matter?" and "What did you say?" and "What are you doing?"
nǐ shuō shén-me
nǐ zuò shén-me
An example question which doesn't end in ma is "你再吃什么？", "What are you eating?".
I'm unclear as to why these questions don't end with the "ma" particle. Is that only necessary for turning a declarative statement into a yes/no question? Is it not used for all questions of the forms who, what, why, etc?