So-called fussy work is said to be fatiguing because it consists of several different rhythms and different degrees of effort and precision. This leads to interference. This is also true of mental work. For example, if one must rapidly alternate between addition and subtraction a greater decrease in output will occur than from performing either one alone. Such work requires a rapid "shift of set." However, a purely homogeneous mental task produces a more rapid decrease in output than do varied or heterogeneous tasks, provided the tasks are devoid of the factor of interference. Similarly, homogeneous or monotonous muscular work is said to be "fatiguing" because it is boresome. If such work with practice becomes so semiautomatic as to permit conversation, boredom should disappear under healthy social conditions in the factory or office. Witness the lady with her knitting. Rapidly performed monotonous mental and muscular work is believed to be associated with a decrease in output, because the same neural patterns are used over and over again without sufficient rest to provide recuperation.