Here's how a biochem friend of mine explained it to me : when you experience something, some set of your neurons fires. If you keep firing that set of neurons, like when you practice something or try to memorize something by reading it repeatedly, those neurons get used to being fired together. When you say the alphabet, if you start saying a,b,c,d... the neurons that fire when you say e,f,g... will fire automatically because they're used to being used right after the a,b,c,d ones.
