Only the luckiest cucumbers get to become pickles, and the crispier the cucumber, the more likely it is to get chosen by the discerning canner. The short answer as to what turns a cucumber into a pickle is a good dose of vinegar. But you can't just pour vinegar on the cucumber and expect it to become a pickle by magic. There is quite an involved process before this happens, especially when the pickles are canned in small batches.
The cucumbers are washed and sliced lengthwise. Though if they are tiny baby cucumbers, they can be used as is. They just have to be small enough to fit comfortably in the canning jar.
The pickle maker fills a large pot with water, puts it on the stove and boils the canning jars, lids and rings to sterilize them. This helps to prevent the pickles from being contaminated with bacteria from their containers. Without this step, they would get moldy pretty fast.
The pickle maker takes another pot and fills it with vinegar and a packet of pickle mix powder (which can be found in most grocery stores) or spices, such as dill or mustard seed. The amount of vinegar used depends on the instructions required by the mix or the recipe used. Different mixes will make different flavors of pickles. The mix and vinegar is heated up until it reaches a temperature that is near boiling but not quite a rolling boil (when bubbles are just starting to form).
Jars are Filled
The cucumbers are put into the jar and the rest of the jar is filled with the hot pickle mix solution. The closer to the rim of the jar, the better. This helps for an air-tight seal. The lids are put back on the jars and the rings are tightened. The pickle maker only gets one shot to put the lid on the jar or else the seal will be broken. Then the jars are immersed in boiling water according to the pickle mix or recipe's directions, usually for between five to 10 minutes.
The jars are put on the shelf and the flavor settles into the new pickles after a few weeks. Then they can be tasted and enjoyed. Since the pickles are sterile and free from bacteria, they'll be good until they're ready to be eaten--even if it is a year later. The jar will need refrigeration after it is opened though.