For each answer you get right, 10 grains of rice are donated through the UN Food Program to end hunger. FreeRice has a custom database containing knowledge questions at varying levels of difficulty. FreeRice automatically adjusts to your level. It starts by giving you questions of increasing difficulty and then, based on how you do, assigns you an approximate starting level. You then determine a more exact level for yourself as you play. When you get a question wrong, you go to an easier level. When you get three questions in a row right, you go to a harder level.
Check it out: http://www.freerice.com/