The best way to describe the technologies we are about to learn is to compare them with what you can find in any self-service restaurant. In such restaurants, you start by picking up a tray, and when you are done eating, you put the tray back. Now imagine a slightly different scenario. You are going to clean the tray before you put it back so that you can put it back on top of the same stack where you took it. The other difference you have to imagine is that, instead of having a multitude of trays that are all equally boring because they all look identical, every tray will have a different picture on it.

So, all these trays are part of a single stack. When you approach the stack, you can only see the picture that is on the top tray. If you add a tray to the stack, that picture will disappear as it is replaced by the one from the tray we place on top. We call that action push. Some old fashioned stacks have built-in springs, so you really have to push the tray down...