FreeRice automatically adjusts to your level of vocabulary. It starts by giving you words at different levels of 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 word wrong, you go to an easier level. When you get three words in a row right, you go to a harder level. This one-to-three ratio is best for keeping you at the “outer fringe” of your vocabulary, where learning can take place.
There are 50 levels in all, but it is rare for people to get above level 48.
Here’s a screenshot of me hitting level 50:
I did not cheat, by the way, though I did have to guess quite a few, or use my deductive powers. It helps if you’ve read a lot of really old books. Trilby? Caleche? No problem.
Murrain means plague, in case you care. I stayed at 50 for a while, then missed a couple, then got back up to 50. I rarely drop below 48. Some of the definitions are odd. I would not, for instance, say that “colobus” means “monkey”. It’s a type of monkey. But whatever :)