Identify a metaphor used in the given article.
One of the metaphors used in the article is comparing self-discipline to a muscle, as mentioned in the sentence "Like a muscle, your self-discipline grows stronger with regular exercise."
This is an article about the importance of self-discipline (https://www.psychologytoday.com/us/blog/chaos-and-control/201809/self-discipline).
Below is an instruction that describes a task, paired with an input that provides further context. Write a response that appropriately completes the request.

### Instruction:
Identify a metaphor used in the given article.

### Input:
This is an article about the importance of self-discipline (https://www.psychologytoday.com/us/blog/chaos-and-control/201809/self-discipline).

### Response:
One of the metaphors used in the article is comparing self-discipline to a muscle, as mentioned in the sentence "Like a muscle, your self-discipline grows stronger with regular exercise."