When Did it Rain?

When my son was in pre-school he traveled by private van to his school in downtown Brooklyn. Since we lived on a dead-end block in order not to inconvenience the driver we would wait for him to call us and tell us he was at the end of our block. Then we would run out of the apartment and down our walkway. During the running my son would often trip on a defect in the sidewalk and fall flat on his face. But he would jump up and continue running to the van. One morning he was running down the walkway and fell flat on his face into a "rain puddle". He just lay there, finally he lifted his head and looked at me astonished and said, "When did it rain?"!