An athiest friend asked me about the story of Jonah being eaten and thrown up by a fish. She said that this seemed imposible. How can I explain this to her?
Your friend is right that the story of the swallowing and later regurgitation of Jonah alive on the third day is impossible. That is the whole point. This is a miracle. A miracle, by one definition, is an event which is "impossible" according to the laws of nature. A miracle is an event which requires the intervention of a supernatural power. We call this supernatural power God. The crossing of the Red Sea was "impossible." So was the healing of the man born blind (John 9), the crossing of the Jordan at flood stage (Joshua 4,5), the resurrection of Lazarus and of course the resurrection of Jesus. Many events in the Bible are clearly represented as miracles, not as natural events. By the way, I have a fairly recent book published titled "That You May Believe." It is on this subject–the miracles of the Bible, and especially the miracles of Jesus (available at www.ipibooks.com).
Some will say that the story of Jonah is impossible and therefore is a complete fiction. This claim amounts to a claim that the miraculous simply cannot and does not happen. Such people may be correct. You should investigate this idea for yourself. However, you should be aware that such people are making an assumption. In other words, they declare a priori that the supernatural does not exist, and then seek to prove that the story of Jonah did not happen because miracles do not happen because of their assumption. This is circular reasoning. Whether or not the supernatural is real is something for us to investigate. However, you cannot disprove it by simply assuming it does not exist.
So, I believe the story of Jonah is not a fiction. I believe it happened. I believe this was a miracle on par with the resurrection of Jesus which was equally impossible by natural means. The difference between the two is that the evidence for the resurrection of Jesus is very strong, while the evidence for the miraculous salvation of Jonah from the whale is, admittedly, weak. I accept this story to some extent on faith in the inspiration of the Bible.