picture shows a shower head with an on/off
control. This is convenient, but together
with the shower control below presents a potential
problem. The problem is that both controls have
to be turned on or the shower won't work. If
either one or the other or both are turned off,
the shower won't work. There really isn't
any good way to tell which one needs to
be turned on. Thus, there are three ways
for the shower to fail to work and only
one way for it to succeed!
design a set of controls, try to minimize the number
of ways for them to fail to work and maximize the number
of ways for them to succeed.