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This paper reports on liquid (deionized water) microlenses that are intrinsically formed and integrated within microfluidics through pneumatic manipulation of fluids inside microchannels. Such microlenses are formed via liquid-air interfaces of liquid droplets, which are pinned at T-shaped junctions of channels. In addition to being tunable in focal lengths (a few hundreds of micrometers to infin) along the microchannels parallel to the substrate used, these microlenses can uniquely be repositioned, removed, and reformed at predetermined locations of the T-shaped junctions within microchannels on demand under pneumatic controls. The design and formation of a microfluidic channel network for the in situ formation are first described. Then, the pneumatic control of the fluids, including the formation, movement, and size control of a lens droplet, is discussed, and the in situ formation processes for single and multiple liquid microlenses are described. The in situ formation processes for a single microlens and a two-lens combination only take tens of seconds, eliminating various conventional microfabrication processes and multiple layers of materials. Finally, the detail of characterization of these microlenses is given.