How much does a prayer weigh?

There is a story of a grocery store owner who tried to weigh one. A tired-looking woman came into the store and asked for enough food to make a dinner for her children.

The grocer asked her how much she could spend. The frail woman answered, "I have nothing to offer but a little prayer." The storekeeper was not very sentimental nor religious, so he said, 
half-mockingly, "Write it on paper, and I'll weigh it."

So she did. The grocer placed the prayer on the weight side of his old-fashioned scales. Then he began piling food on the other side; but to his amazement, the scale would not go down. He finally became flustered and gave the woman a large bag of food.

The grocer never saw the woman again, but he treasures the slip of paper upon which the woman's prayer had been written;

"Please, Lord, give us this day our daily bread."