But it is far from infallible. It sometimes distorts the facts, and it tends to become less reliable as we age. Long-term memory as a whole is defined by the criterion of long duration. But other criteria can be applied to break down the complex phenomenon of memory into separate components. One such criterion is whether or not the long-term memory in question can be verbalized. On the basis of this criterion, two main forms of long-term memory can be distinguished. The first is declarative memory : your memory of all those things that you are aware of remembering and that you can describe in words, such as your birthday, or the meaning of the word "cradle", or what you ate last night. This form of memory is also called explicit memory, because you can name and describe each of these remembered things explicitly. The other form of long-term memory is non-declarative memory . It is also known as implicit memory , because you express it by means other than words. For example, when you ride a bike, juggle some balls or simply tie your shoelaces, you are expressing memories of motor skills that do not require the use of language. Such "motor memories" are just one type of implicit memory. There are others as well .