Also you use little cucumbers to make pickles, not the big ones that you would chop up to use in a salad.

Cucumbers can only change into a pickle if someone makes it happen, but they don't do it on their own.

Pickling, is the process of aging an ingredient in a solution (liquid) that is high in acidity (sour flavor) and salinity (saltiness). It is a fairly complex process, that can allow the user (eater) a much longer time table in which the pickled food is edible. It can be done fast (pickled onions) or slow (pickled cucumbers), and as nutcakes said, it really is quite simple to pickle your own foods. Vegetables, fruits, even protein (eggs, in this case) can be pickled, and as a result their flavor will become more sour.

Pickles are made from cucumbers, but you can also make pickles from other things. Maybe you have not had picked carrots or green beans, but I bet the store you are in sells them. You can also make them. It's easy. Just canning them is the more complicated part.