What can be put in a bed under 20 mattresses and 20 eiderdowns to find out whether a person is a real princess or not?

Explanation

The fairy tale "The Princess and the Pea" is written by the Danish author Hans Christian Andersen. In the fairy tale the queen places a pea in the bed where the princess is to sleep. On top the queen puts 20 mattresses and finally 20 eiderdowns to find out whether the girl is a true princess or not. Hans Christian Andersen also wrote the fairy tales "The Emperor's New Clothes," "The Tinderbox," "The Ugly Duckling", "Thumbelina" and "The Little Match Girl".