When someone starts drawing, they rarely make use of varied lines even though they are a powerful took to impart originality.
The idea is: Not all lines are equal. Small changes in the width and darkness of your lines can create a dynamic impact on the picture and make it more visually interesting. However, this is easier said than done. Changing the lines requires a lot of practice.
Experiment with different pencils ranging from 3H to 6B and holding the pencil in different angles to get the desired results. With some practice, you will be able to figure out how changing the depth of one line can impact your whole drawing.
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