Why Ganesha has an elephant head?

Ganesha, it hardly needs to be mentioned, had a ravenous appetite. Witness his huge belly. One day, he was invited out to a feast and stuffed himself with a popular sweetmeat called laddoo. Having had his fill, he set out for home, late at night, on his favourite mount, the mouse. Suddenly the mouse wobbled... he had seen his mortal enemy, a snake, on the path and had become petrified. As a result Ganesha fell off, his stomach burst open and all the laddoos rolled out. Unperturbed, the elephant-headed God got up, stuffed the laddoos back in and girded his belly with that self-same snake.
However, seeing all these goings-on, the moon could not contain itself and had a hearty laugh. But this action offended Ganesha. He promptly cursed the moon, causing it to disappear from the heavens.
Alas, poor earth, she wilted without moonlight pouring over her at night. So the gods appealed to Ganesha to reconsider his curse, and a penitant moon added its own bit by apologising. So, a mollified Ganesha modified his curse making the moon invisible only one one day of the month and causing ill-luck to befall anyone who sees the moon on his feast-day.