A prince has been unable to find a princess he likes well enough to get married to. One stormy night, a woman shows up at the castle, claiming to be a princess. She is beautiful, and the prince smitten with her, but the queen decides on a little test, and places a small pea under twenty mattresses in the princess's bedroom.
In the morning, the queen asks the princess how she slept. The princess claims she is all black and blue, and didn't get a wink of sleep from tossing and turning, because her bed was lumpy. In this way, the queen knows she is a real princess, because she is so sensitive.