While I was little, I was interested in everything, and I wanted to know why.One day, my mom gave me a cup of water to drink, I put a spoon in the cup, and found that the spoon was not as straight as it before. What happened to my spoon? Was there a ghost? And after taking it out, the spoon was straight again; but put in the water, it changed again... I didn't know the reason and I was looking forwards to getting the correct answer. Waiting and waiting, until my old cousin told me this physics question.