(a) General. Work experience means skills and abilities you have acquired through work you have done which show the type of work you may be expected to do. Work you have already been able to do shows the kind of work that you may be expected to do. We consider that your work experience applies when it was done within the last five years, lasted long enough for you to learn to do it, and was substantial gainful activity. We do not usually consider that work you did more than five years before the time we are deciding whether you are disabled applies. A gradual change occurs in most jobs so that after five years it is no longer realistic to expect that skills and abilities acquired in a job done then continue to apply. If you have no work experience or worked only “off-and-on” or for brief periods of time during the five-year period, we generally consider that these do not apply. If you have acquired skills through your past work, we consider you to have these work skills unless you cannot use them in other skilled or semi-skilled work that you can now do. If you cannot use your skills in other skilled or semi-skilled work, we will consider your work background the same as unskilled. However, even if you have no work experience, we may consider that you are able to do unskilled work because it requires little or no judgment and can be learned in a short period of time.