Boil water first when cooking what foods? The directions on some packages of raw food (frozen vegetables, dry beans, dry pasta, rice, and the like) indicate that one should first boil water, then add the food, wait for the mixture to boil again, turn the heat down, and cook the stuff for some length of time. The directions on other packages indicate that one should combine the water with the food first, bring it to a boil (or to the boil if you're British), turn the heat down, and cook the stuff for a length of time. (That is, the latter directions don't include boiling the water first: the water is heated with the food in it.)

What determines which types of food are cooked each way? Or is one set of directions right (for all foods) and the other wrong?
There is no single universal answer to this question, as it depends on the specific food and outcome desired, but there are a couple of common themes that influence whether starting from cold water or boiling water is the preferred method:


Larger foods that need to cook through may overcook on the outside before the inside is cooked, if started in boiling water.  For example, potatoes are usually started in cold water so that they cook through to the center.
Some foods contain enzymes that will be deactivated at boiling temperatures, but may act more quickly at lower temperatures.  Starting in boiling water would be for when it is desirable that the reactions be inhibited; starting in cold water facilitates the reactions continuing. 
Sometimes it is simply for precision to get a repeatable recipe.  Boiling water is always the same temperature (at least for a given altitude), so beginning cooking at the boil is reliable and repeatable compared to starting with cold water which may have an unknown temperature curve as it comes up to the boil.