Here's why:
* Collective Noun: A collective noun refers to a group of individuals or things considered as a single unit.
* Examples: A flock of birds, a herd of cows, a team of players, a pack of wolves.
* In this case: A pack of pencils refers to a group of pencils, treated as a single entity.
Therefore, "pack" is a collective noun in the phrase "a pack of pencils."