How come the kettle starts to boil and steam comes out of the kettle before the kettle finishes and reaches 100 degrees

Question: How come the kettle starts to boil and steam comes out of the kettle before the kettle finishes and reaches 100 degrees celcius?
- Keywords:
  100%,
  kettle,
  reaction,
  water
Asked by thoma058 to Emma, Jimmy, Janet, Niall, Simon on 13 Mar 2013.
- James Holloway answered on 13 Mar 2013:
  
  Water turns to steam at 100 degrees (at atmospheric pressure).
  
  When you put a thermometer in hot cup of water you are measuring the average temperature of that water. If the thermometer reads 90 degrees that means that some water molecules will be at 80, others at 100. It is the ones at 100 degrees that you see coming off the hot cup of water as steam.
  
  Thats why you see steam coming out of a kettle early – because the average may be near 100, but many individual molecules are over 100 degrees and they boil off.

Question: How come the kettle starts to boil and steam comes out of the kettle before the kettle finishes and reaches 100 degrees celcius?