Ask somebody who has no truck with e-books why they don’t like the medium, and they’ll usually list the qualities a physical book has that are missing from the digital experience: the heft of the book in their hands, beautiful cover art, the feel of the pages, and the smell of a real book. That old book smell is instantly recognizable to anybody with a passion for reading, but where does it come from?

IO9 has posted a very informative video from AbeBooks explaining exactly how books get that musty scent, and it turns out the answer is mainly chemical. Scientists at the University College London investigated its source, finding that many books release organic compounds into the air — from the wood pulp that makes up their pages as well as the ink — that give older books their unique mustiness, along with what the lead scientist described as “a hint of vanilla.”

The classic book smell is a result of decay, brought on by acidity, and pages with high acidity are actually common in works from the 19th and 20th centuries. The earliest books were printed on paper with higher purity, and have thus staved off decay for hundreds of years — so referring to it as old book smell is actually something of a misnomer.

Check out the full explanation below: