Questionaut

Questionaut is a short educational game made by Amanita Design. It's a point-and-click adventure game where you have to control an aeronaut who tries to get with his balloon as high as possible to get a hat which belongs to his friend.

You have to solve a simple problem and then answer questions. When you give the right answer, the balloon gets some air but if you give the wrong answer the balloon loses its air. Journey through strange worlds and test your knowledge of English, Maths and Science on this magical mission to recover your friend's hat.

When you're stuck and want to know how the story evolves, there's a complete walkthrough available.

(thanks Linda)