Here's why:
* Simile: Uses the words "like" or "as" to compare two things. In this case, "watching the show is like watching grass grow."
* Metaphor: Directly compares two things without using "like" or "as." It states that one thing IS another.
* Analogy: Explains something unfamiliar by comparing it to something familiar. While it can use similarities, it often focuses on a more complex relationship between the two things.
Since the sentence uses "like," it's clearly a simile.