Sentence:  It is made up of tumbling , which might be said to start with a somersault , run through such stunts as headstands , handstands , cartwheels , backbends , and culminate in nearly <WSD>impossible</WSD> combinations of aerial flips and twists and apparatus work . 
Meaning of the word marked with <WSD> tags:  not capable of occurring or being accomplished or dealt with