It is possible for something to be impossible, depending on the context in which you use the phrase. For example, Saturn is a planet that fascinates humans, but it is impossible for anyone to land on its surface for exploration. The temperatures at its core can reach as high as 21000 degrees Fahrenheit.
It's just a figure of speech. It is impossible for a human to fly without any mechanical by simply flapping his/her arms. It is impossible jump out of an airplane at 4000 feet with no protective equipment and survive the landing.