The story runs as follows:
Once Shri Gauri asked her two children to go around the Earth and she
promised a present to the one who will come first. Shri Kartikeya
immediately mounted his peacock and started off. But Shri Ganesha had
only his little mouse and he understood that he would never come
quicker than his brother. Then an idea occurred to him, “What can be
greater than my Mother as it is She who has created the Earth”. And he
mounted his mouse and made a circle on it around his mother.
Shri Ganesha won the competition.