How do they assemble the cranes that are used to build high rises?

Question

I just can't figure this out. They need a crane to assemble tall buildings, but how do they assemble the crane? Especially if the building is over 20 stories high. What crane can build a crane higher than itself?

Answer 1

a mobile crane sets the original lower sections in place, and then the crane is assembled in 6' sections as it climbs itself.