A tiny opening for a dangerous accident
A pen cap is small enough to be swallowed accidentally, especially by a child. If it lodges in the throat, it can block the airway. A solid cap could seal that passage completely; a cap with a hole may leave a route for some air to pass through.
That opening is a safety feature, not a guarantee. It may reduce the danger of total airway blockage, but a lodged cap can still cause choking and needs urgent attention.
Why the hole matters
The hole keeps the cap from acting like a plug. Its benefit is simple: if the cap ends up in the airway, an opening can allow limited airflow while help is sought. The design does not prevent swallowing, and it cannot ensure enough air will get through in every situation.
The useful detail is easy to miss: a small hole can change what happens when a tiny object becomes an obstruction—not by making the object harmless, but by leaving a possible path for air.
