How I Use It
They have a quiz section on periodic table elements and names. Since I teach a unit on the periodic table, this is perfect for a 5-10 minute filler if an assignment is finished early in the computer lab. I wouldn't count on it keeping their interest for much longer unless you gave them a goal to accomplish such as earn enough rice to feed 10 people or something. I would be careful since it is ad-supported you never know if a kid/parent has an issue with a product being advertised so you might want to make it optional or extra credit. I use it as an option with other sites that are fun/educational to keep them engaged for a few minutes if they finish the main assignment early.
Free rice has multiple choice style questions that are ad-supported but the money earned apparently goes to helping provide rice for under-nourished people. The students get feedback on how much rice they earned and how many people they fed. I haven't seen any inappropriate ads but you should know that it is ad-supported.