Learning is through associating new information with old information. We establish our knowledge about our world integrating every input factor. I think sometime it is unreasonable.
Nowadays cell phones have this function that people can make a stream of music as the sound of alert or ring tone. I am interested in this function and set one of my favorite songs as alarming sound. But it is a failure and I will never do it again. Because every time I listen to this song I am in a sleepy state and hoping to sleep more, I feel frustrate when I listen to this music. Then after several trails, whenever I listen to this music, I am elicited a noxious emotional response. That is to say, I begin to dislike this song. My brain adds a noxious tag to the innocent song. This is an example of emotional conditioning.
This memory is also flexible and can be extinguished. After I recognize that I don’t like the song, I intentionally listen to this song when I am happy. Then the noxious tag is slower replaced by an amiable tag, I like this song again. This is an old memory covered by new memory, in other words, recognition.

我觉得其实学习东西是一个奇怪的过程。

比如一开始手机有下载音乐和自由设置铃声的功能，我觉得很好玩。就把一首我很喜欢的歌设为闹铃声。结果，呜呜，在每天早上带着惺忪双眼听到的音乐，逐渐成为了我最讨厌和反感的声音。于是只好把这个铃声换掉，然后无论我换那首歌，作为闹铃声后一段时间，它都让我讨厌。

所以，其实大脑挺不讲道理的，听到这首歌不幸刚好在心情不佳的时候，于是这首无辜的歌也被大脑加上了厌恶的标签，但其实，厌恶的是早起和美梦无踪而非这首歌本身。

这就是巴普洛夫的条件化学习。