The old woman put before the young man an empty bowl:

The bank is empty, right?

and yes.

And now? Using large stones, she asked.

“Now the bank is full,” the young man replied.

Smiling, the old lady poured sand into her and questioningly looked at the guy. He reluctantly sneered, agreeing to some errors in his previous words.

Smiling even wider, the old lady took a glass of water and began to pour it into the already seemingly filled bowl. The boy could not withstand:

“Yes, grandmother, you are right: the bank was never full. But stop telling me, moving the half empty plate away from me, I am really already full.