What Makes Our Game Difficult?

What makes Klangfarbenmelodie's game difficult?

I don't think our game is difficult in any sense, but it is still based around what I would assume to be an approved STEM concept. The questions are fairly straight-forward, in no sense esoteric. The first mini-game may require a bit of time to experiment, but I'm sure any student could pass it. The fun is in the second mini-game. And, well, there's fun in the third mini-game too. I don't think the aim of our game is to be difficult, but to be informative or reinforcing of knowledge that has already been acquired.