Free Rice!

Let me recommend the online vocabulary improvement game, Free Rice. Not just a game, but a way of donating rice to the hungry of the Third World. For every word you get right, 20 grains of rice are donated. I got up to 2,400 grains before I packed in, so you can tell that it is rather addictive. There are some difficult words in there, but, very often you can select the right meaning because all of the other meanings appear to be wrong! What the 'vocabulary level' might be is not explained, but I guess it has something to do with the number of words you get right.
Currently donations stand at over nine thousand million (or nine billion in American) grains of rice donated. That sounds a lot, but it would be nice to see it translated into kilos.