Why do people yawn? No one seems to know the exact (1)________. Some people believe that we yawn (2) ________ we do not have enough oxygen in our bodies. In (3) ________ words, we yawn to breathe in (4) ________ oxygen. People also believe that yawning is associated (5) ________ boredom.

Select the most appropriate option to fill in blank number 3.

The correct option/answer is other.

Let's analyze the context of the sentence containing blank (3) to understand the correct choice:
The sentence reads: "In (3) ________ words, we yawn to breathe in (4) ________ oxygen."

The phrase "In other words" is a standard idiomatic English expression used to introduce an alternative, simplified explanation or paraphrase of what has just been stated. In this paragraph, it connects the previous statement ("we yawn because we do not have enough oxygen") to the explanation ("we yawn to breathe in more oxygen").

Let's evaluate the options to see why they do not fit:
- some: The phrase "in some words" is not grammatically correct or idiomatic in this context.
- new: "In new words" is not a standard expression for rephrasing an idea.
- rather: "Rather" is an adverb and does not fit grammatically in the construction "In [adjective] words".
- other: Completes the standard transition phrase "In other words".

Therefore, the most appropriate option to fill in blank number 3 is other.