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A standard biro pen was the first pen many Brits learned to write with in primary school.
Some of us would chew the lids or even pretend they were whistles by blowing down the little hole, which a lot of people didn't know had a "life-saving" function.
While many of us dismissed it as "something to do with the ink" – the hole is actually there for a much more important reason.
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Zack D, who regularly shares discoveries he finds interesting, took to Instagram to his 213,000 fans the pen companies are legally required to add these tiny holes in the design.
In the clip, which gained more than 3.1million views, he explained: "Have you ever wondered why pen caps have those little holes in them?
"Well, most people think it has something to do with the ink, but the actual reason is much more important than that.
"Pen companies are actually required to add these little holes.
"You see, it's just enough space for airflow if the cap gets stuck in someone's throat."
Users took to the comments shocked by the revelations as they realised just how often those pen lids were in their mouths as a kid.
One user wrote: "I use them as little whistles."
Another added: "I remember chewing this pen cap during my childhood."
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A third said: "Wow hopefully that never happens to anyone."
However, others were less than impressed as they pointed out that not all pen brands have the hole.
"Not all pen caps have holes.," one wrote.
Another added: "I thought this was common knowledge."
A third commented: ''Same reason with lego head that always has hole in it.''
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