The game, FreeRice, tests the vocabulary of participants. For each click on a correct answer, the website donates money to buy 10 grains of rice.
Companies advertising on the website provide the money to the WFP to buy and distribute the rice.
FreeRice went online in early October and has now raised 1bn grains of rice.
It goes on as long as you let it, apparently, and I got sucked in for quite a while. I got up to level 46 - and as low as 35 (not good) - and donated 4,780 grans of rice. To someone.