What did the Queen place in the Princess's bed to establish her royal ancestry in the fairy tale by Andersen?

"The Princess and the Pea" is a literary fairy tale by Hans Christian Andersen (2 April 1805 – 4 August 1875) about a young woman whose royal ancestry is established by a test of her sensitivity. The tale was first published with three others by Andersen in an inexpensive booklet on 8 May 1835 in Copenhagen by C. A. Reitzel.

The story tells of a prince who wants to marry a princess but is having difficulty finding a suitable wife. Something is always wrong with those he meets and he cannot be certain they are real princesses because they have bad table manners or they are not his type. One stormy night a young woman drenched with rain seeks shelter in the prince's castle. She claims to be a princess, so the prince's mother decides to test their unexpected guest, by placing a pea in the bed she is offered for the night, covered by huge mattresses and 20 feather-beds.

In the morning, the guest tells her hosts that she endured a sleepless night, kept awake by something hard in the bed that she is certain has bruised her. With the proof of her bruised back, the princess passes the test and the prince rejoices happily, for only a real princess would have the sensitivity to feel a pea through such a quantity of bedding. The two are happily married, and the story ends with the pea being placed in a museum, where, according to the story, it can still be seen today unless someone has stolen it.