I liked the game being educational. Also the music and sound effects are matching well!

Very educational. It seems I did forget quite a bit of history facts. From a user interaction design perspective, it doesn't make much sense that the player has to click on the paper to see the contents. Just show it on the screen directly when the level starts. 

yeah!! i liked the idea behind it!! it's very unqiue and awesome, i love it!!

What a fun way of implementing the theme! I love that concept, however actually i have one thing to note (maybe)

I am from the united states, and we are taught the "Discovery of the Americas" pretty often. However a few things about said discovery was:

1. Vikings were actually found to have visited the americas in the 1,000s, much earlier than columbus.
2. There were already people living there. this kind of gets into what it counts as discovering a land, because while it was new information to columbus and europe, it was already known by entire swathes of people who lived there.

I think this error is actually wonderfully tied into the theme (and fits in the game perhaps), where maybe 100 years from now we might look at pieces of history and know them to be untrue, things that we all think are true in the current day.

Honestly though, this is such a fun idea. i love the format of thinking about if something was right, or what could be wrong about it, and learning history in the process. the sfx were also well implemented, and the feel of using the stamps was just right. Great work!