A computer can be built out of any
bi-stable device. What I mean by this is that the elements of the
computer must have two stable positions. These two stable states
are ON and OFF, and represent 0 and 1.

The basic building block of most
computers today is the transistor. A common comparison used for
understanding is the light switch. Much like a switch has two
positions, on and off, so does the transistor. The OFF state means
no electrical current is allowed to flow through. ON means that
current can flow through unimpeded. The change to ON or OFF is
done electrically and not mechanically, so the speed can be quite
fast. At the same time, they can be very small. A transistor can
change states in roughly 3 billionths of a second and you can fit
roughly 10 million transistors into a space 1 cm area.

Transistors are made from
materials called semiconductors, such as silicon or gallium
arsenide. A huge number of transistors and the electrical pathways
in between them can be printed like a photograph onto a wafer of
silicon to produce an integrated circuit. It is this that forms
the construction of computer chips. The design of these circuits
is actually pretty cool. They use tightly focused light to to
write the designs into silicon wafers. These designs can be kept
like photos and are used to mass-produce chips of the same design.