Beard hairs in books

Somewhere in his sprawling Minhage Yisrael, Prof Daniel Sperber discusses the custom of readers leaving the hairs that fall from their bears within the pages of the book they are reading. Kabbalistic reasons have been given. But now we know the real one - to preserve their identity for posterity:

Scientists have been examining Copernicus’s skull and leg bones since they were discovered by archaeologists three years ago in an unmarked grave in the cathedral. Testing revealed that the body was of a 70 year old man, and that he had a broken nose, which would have the same age and condition of the Polish scientist when he died. Furthermore, DNA taken from teeth and bones matched hairs that were found in one of his books, and they ultimately concluded that they had found Copernicus.

From here.