I just found this info on their website’s FAQ page. Interesting:
FreeRice has a custom database containing thousands of words at varying degrees of difficulty. There are words appropriate for people just learning English and words that will challenge the most scholarly professors. In between are thousands of words for students, business people, homemakers, doctors, truck drivers, retired people� everyone!
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.
I actually got to level 50 once. Mostly I hover between 46 and 48. The funny thing is much of the time I’m guessing. But I tend to be a very good guesser. I suppose some of it is knowledge of Latin roots and also some awareness of what suffixes indicate about a word’s part of speech. But sometimes I wonder how the heck I knew what the right answer was. And I did know it, I wasn’t just randomly picking. Wow, I really am a word nerd.