When play starts, you see the results of your first throw of the dice.
Click on the dice you wish to roll again and they will turn into
pumpkins -- thanks Tigert :)
When you have selected all the dice you wish to reroll, press
the "Roll!" button.
When you have a hand you like or have run out of chances to
throw the dice, click on a row corresponding to what you think
is your best scoring combination.  It is very possible to have
a score of zero and if you click on the wrong row, you
will be scored accordingly. (Say for example, you get
yahtzee -- 5 of a kind -- and click on the large straight row,
you will receive a zero in your large straigt score).
You can only use each combination once, i.e. once you've entered
something in a slot, you can't change the score later if you roll a
better hand.
