Automobile ignition coil (spark coil)
It is possible because the ignition coil is not a "normal" transformer


  An ignition coil is essentially an autotransformer with a high ratio
  of secondary to primary windings. By "Autotransformer", I mean that
  the primary and secondary windings are not actually separated - they
  share a few of the windings. The ratio of secondary to primary turns
  in an ignition coil is somewhere around 100:1. The ignition coil is
  operated directly off a 12 volt source. However, the ignition coil
  does not work like an ordinary transformer. An ordinary transformer
  will produce output current at the same time that input current is
  applied. An ignition coil actually does most of its work acting as an
  inductor. When the ignition coil is connected to the battery, the
  inductor is 'charged' with current. It takes a few milliseconds for
  the current to build up the magnetic field - this on account of
  reverse voltage caused by the increase in magnetic field. During this
  short charging period, maybe a thousand volts are produced at the high
  voltage terminal, not enough to actually cause a spark.
  
  The actual spark is generated when the breaker contacts open. For an
  ideal inductor, the current and voltage relate by:
  
  V = L dI 
  
  Where V is voltage, L is inductance (in henrys) and dI is the
  rate of change of the current.
  
  Thus, seeing that L is constant for the inductor, the abrupt change in
  current will cause a very large voltage to be produced. This produces
  a very short, very high voltage spike. Of course, the change in
  current is on the primary side, but because the primary and secondary
  coils have a large mutual inductance (this is where the transformer
  part comes in), you get a spike on the order of 100 or more volts on
  the primary, and 10000 volts on the secondary. Even the primary side
  of the coil can give you a bit of a jolt if you hold the wires while
  disconnecting power. Also note that any contacts you will use will get
  a lot of sparks, also on account of this.


From http://www.sentex.net/~mwandel/cannon/sparky.html