Hit wicket

The striker is out Hit wicket if the wicket is put down by either his bat or his person. It is a form of dismissal that should be very easy to judge but can often prove to be quite tricky in practice - since the decision whether he should be given out is dependent on exactly when and under what circumstances the wicket was hit.
Firstly, the ball must be a fair delivery. Secondly the Law comes into action while the ball is in play but after the bowler has entered his delivery stride - in other words just before the ball has even been released, far less reached the striker.
If the striker puts his wicket down - perhaps by treading on it - at any time after the bowler has entered his delivery stride until he has played the ball, or at least has had that opportunity to do so, he should be given out. The same should apply if he puts the wicket down if he immediately starts to run having hit the ball or even having not attempted to play the ball - providing that run, too, is started immediately after he had the opportunity of playing it. He should also be given out if he puts his wicket down while making a second or subsequent stroke in an attempt to stop the ball hitting his stumps.
The key question for the umpire to answer is whether the striker set off for his run immediately after playing the ball or having the opportunity of playing it. If he paused for an instant - perhaps to see where the ball went - then slipped while setting off and dislodged a bail, he should not be given out.
If the striker breaks his wicket while trying to avoid being Run out or Stumped he might well find himself dismissed in those ways, but not for Hit wicket, nor should he be given out if he breaks his wicket while in the act of running or avoiding a throw-in.

Interestingly, if a piece of wood snaps off the striker's bat or the bat comes apart, and some of it breaks the wicket, the striker should now be given out, since this Law now applies to any part of his bat. This part of the Law was changed in October 2010.

But the striker can be given out Hit wicket in circumstances where he was not in contact with the object that broke the wicket! Examples include a wicket broken by the striker's cap blowing off. In the early days of the game, when players habitually wore top hats, there are entries in scorebooks recording dismissals as 'out - hat fell on the stumps'!