Free rice: Vocabulary online game

Free Rice is an online vocabulary game where students have to find the right definition or synonym of the given word.

For each correct answer, the site donates 10 grains of rice through the World Food Programme to third-world countries.

This is the description Free Rice makes of their project: 

"Free Rice is a non-profit website that is owned by and supports the United Nations World Food Programme. Free Rice has two goals:

• Provide education to everyone for free.
• Help end world hunger by providing rice to hungry people for free.

This is made possible by the generosity of the sponsors who advertise on this site."

There are sixty levels of difficulty and you can choose the subject to answer the questions about.