Impossible stories

Each of these stories may seem impossible at first. Can you figure out the solution?

1. A man and his son were on a tour of an atomic power plant. In the control room, the boy asked if he could see the controls for the reactor core. The head physicist said yes, and explained how the controls worked. After the boy left, the head physicist turned to an assistant and said, "That was my son." How could that be?

2. Three kids from Bristol went for a walk. About a mile into the walk, they came to a deep, wide river. There was no bridge. They didn’t have a boat or raft, or any materials to make one. None of them could swim. How did they get across?

3. Lisa walked out the back door of the farmhouse on a Thursday afternoon and found a man’s pipe, a scarf and three lumps of coal lying on the wet grass near the barn. The nearest neighbours lived a mile away, and no-one had visited that day. Where did the objects come from?

4. A true story: a white horse jumped over a castle and landed on a bishop, who immediately disappeared from the landscape. Where did this take place?