Hedgehog rule refers to two sleepy hedgehogs, who are huddled together because of the cold. But because each of them had thorns, they left a distance, but they were too cold to stand, so they got together. After several twists and turns, the two hedgehogs finally found a suitable distance: they can get each other's warmth without being stuck. Hedgehog rule emphasizes the "psychological distance effect" in interpersonal communication.