In this particular mobile, each lever is formed of a single piece of metal bent into a loop and hook at exactly the right place for balance. The fulcrum (loop) is located so that it balances the counterweight, the weight of the wire, and the weight of the sub-mobile that hangs from the hook.
The hook-and-loop construction
makes the mobile behave like a spring when it twists. When a loop twists in
its hook, it climbs up the sides, converting the mobile's kinetic energy
into potential energy as it hoists itself up. Some cleverly designed
mobiles use this to control how energetically parts move relative to each
other. The hook-and-loop construction also moves the fulcrum above the rest
of the wire, which makes it possible to balance the lever at a skewed angle.
(This is why rod-and-string mobiles are always horizontal and dull.)
Hook-and-loop looks interesting, but it is often hard to get the balances
right.
He built
his mobiles from the inside out, first looping and hooking a wire, then
looking for a spot along its length where a weight would balance. If you go
to a museum and you can see traces of his trial-and-error method in the way
he clipped wires and stapled on weights where ever he found a favorable
balance. Occasionally he built two or three models of the same mobile,
trying to find a set of balance points that would allow him to place the
weights exactly where he wanted them to float in space.
Mobile construction
Mobiles are a difficult medium. It can be quite hard to make a
mobile look "as intended." Where the masses will float in space, how
long the wires are, and where the fulcrums wind up are largely dictated by
the mechanics of balance. This is probably why nearly all mobiles are
abstract and not representational.Inside-out construction
Calder said he usually meant to make one thing and wound up making
another. 
Outside-in construction
Alternatively, you can build a mobile from the outside in: Choose where the
weights will float, sketch wires, estimate balance points, and repeatedly
revise the wires until they pass through their balance points. Because the
wires have mass, revisions change the balance points, making outside-in
method rather painstaking. However, because it allows much more control
over the what the mobile will actually look like, it opens the door to a
realm of much more expressive and representational kinetic sculptures. For
example, here is a mobile bestiary I made this
way.
Although painstaking by hand, prebalancing is perfectly suited to a computer artist capable of precise calculations. Of course, this is the only part of mobile design that computers can do with ease. To see how a computer was endowed with the vision, design, and aesthetic sense to do the rest, visit the artificial artist page.