One Markendey Sage wanted to see apocalypse, so Krishn gave the view of the world during apocalypse.

There was water everywhere. No land, no animal, no plant, not a single living being was alive. Everyone and everything was drowned in the huge deluge of water.
Krishn, in the form of child, was alone and resting on a leaf of banyan tree. He was sucking the thumb of his foot.

After the apocalypse was over, sage asked Krishn, 'Why did you put the thumb of your feet in your mouth during the huge deluge?'
Krishn said, 'All my devotees, Brahma and all others wash my feet and drink that water. They must be getting something special and valuable out of it. I thought if everyone is deriving benefit from my feet, why should I be deprived of its speciality? So I put thumb of my foot in my mouth to seek that benefit.'