Why don't all questions have the "ma" 吗 particle at the end? 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?"


  shén-me shì
  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?
Your guess is correct. 吗 (ma) is usually used at the end of a narrative sentence, which without 吗 can become a declarative statement. 

吗 has other usages but at this learning stage you shouldn't let them distract you.