It sounds too good to be a real story (who knows, though), but I like it:
One day a mother came to Gandhi for advice, bringing her son with her.
‘Please,’ she said, ‘will you tell my son that he has to stop eating so many sweets? They’re not good for him, they’re making his teeth rot. I tried and tried to make him stop, but he just won’t listen. I think he’ll listen to you.’
Gandhi shook his head.
‘Come back in a month,’ he said. ‘Bring your son with you.’
One month later, the lady came back with the boy. Gandhi looked at him and said:
‘Young man, you need to stop eating all this sugar. It’s bad for your health.’
The mother was quite surprised to hear this. ‘But, Bapuji, couldn’t you tell him this last month? Surely you didn’t need a month to think about it?’
‘No, I couldn’t,’ Gandhi said. ‘Last month, I too was eating far too many sweets.’