Walnuts

We were all buying some strange stuff at the market. I entered a small bungalow where I wanted to buy magical balls. To check their properties, one had to take them to the other bungalow. There, they started to tremble, roll, make sounds or talk, or even wink (if, of course, a ball was an eye). I bought some but it wasn't enough for the old wicked seller. She was a witch - I knew it for sure - and she could cast a spell on me if I didn't buy some walnuts as well. I took a small bag of them. I crushed one of the walnuts. It was black and mouldy but I didn't care much and started to munch.
It is hard to describe the place we were exploring. I think it was a kind of dungeon. I was puzzled because I knew we were to have an exam in a minute and I wasn't prepared well. I tried to rationalise it a bit, convincing the others (and myself), that professor K. had no right to demand the knowledge of the all 'Harry Potter' books from us. I was sure the exam wouldn't be based on the details but rather on practice. At the same time I knew that all of my friends knew 'Harry Potter' much better than I did. We went into a chamber and looked for something when Kuba started to cry. He was this little guy who I remember from primary school. He fell and touched poison ivi. He had long red strips on his legs. The whole group started to panic and the situation was getting worse. We knew he would suffer from enbelievable pain. I realised that I knew what to do! I had walnuts! Immediately, I spat them out and put on Kuba's leg. He smiled. I did the same with another friend. She also felt better. I was glad I could help them even if I didn't know the whole book. And... because I passed the exam.