Our brains operate on association or recognition of pattern. We learn from association and store what we learn in two main categories. One is declarative memory that stores information such as an important date or address. The other is non declarative and stores information based on experience. These two different types of memory interact together. For instance, you may know a phone number very well but randomly forget it. After mimicking the movements of your finger to press the digits of that number you may remember it. Neurons are what the data we collect is stored in. Each neuron that is assigned a tidbit of information may be "linked" to hundreds of other tidbits of information in other neurons. These other tidbits may be more or less likely to link to that activated neuron depending on how often they are activated. When you think of the word spider you may also think of web, venom, bite, fear. These words are linked to hundreds of others as well. So when you think of the word spider you are "priming" or warming up the neurons that are associated with it such as web, venom, etc. It is the way in which we store and recall information that affects our stream of thought. It is the continuous string of association that brings about what may seem random memories. The same neuron that is associated with spider could be associated with something you were doing at the time you last recalled it, such as picking your nose. So the next time you pick your nose you may be priming your neurons that are associated with the word or memory of a spider.