The Alter Rebbe held his young grandson, the Tzemach Tzedek, in his lap. The child said to his grandfather: "Zeide, Zeide!"
The Alter Rebbe asked him: "Where is Zeide?" The grandson pointed to the head of his grandfather. The Alter Rebbe said, "This is the head, not Zeide." The child then pointed to the Alter Rebbe's heart and said, "This is Zeide." The Alter Rebbe responded, "This is the heart, but not Zeide." The grandson continued to point to the other parts of the Alter Rebbe's body in order to find the place where "Zeide" is. To all these attempts, the Alter Rebbe responded that he had indicated a specific limb of Zeide, but not Zeide.
The child climbed down from his grandfather's lap and began walking around on his own. When he approached the door, he pretended that his fingers had gotten caught in the door and began to yell, "Zeide, Zeide!"
The Alter Rebbe turned to the child and said: "What is it my son; what happened?"
The child replied: "This is Zeide!!"
Otzar Sippurei Chabad, vol. 17, p. 291