On freerice.com you can feed hungry people by finding the right synonym for a word. Each time you answer correctly, advertisers from the site will donate 20 grains of rice to the United Nations World Food Program.

A fairly simple example might be to select another word for perspective out of: limitation, pharmacist, eruption, or view.
If you choose the correct answer view, you have donated your first 20 grains of rice to somebody starving.

The difficulty is permanently adjusted and will match your skills. If you get a question right, you get a harder word. If wrong, you get an easier word.

This site makes you smarter. It improves your speaking, writing, and thinking skills…

It also provides pointers to other sites about fighting hunger and what you can do besides improving your vocabulary!