Sentence:  With them [bicycles], children can accomplish such unique and delightfully dangerous stunts as “<WSD>wheelies</WSD>”—that is, riding at a fast speed with the machine's front wheel raised completely above the ground. 
Meaning of the word marked with <WSD> tags:  An action or stunt where a bicycle, motorcycle, or other vehicle is ridden for a short period while it is standing only on its rear wheel or wheels.