What Kathy said and ...
This depends on exactly what you want to do and what version of
PowerPoint you have.
If you want a sentence to appear first (with a blank) and then on mouse
click, the answer to appear, a simple animation in any version of
PowerPoint will work (either have the box exit or a textbox with the word
enter.
If you want the word to come in when you want it (and not necessarily on
the next mouse click after the sentence) or you want more than one word
in no particular order, you will want animation triggers (only available
in 2002 or 2003).
If you want the above scenario with an earlier version of PowerPoint or
you want an answer specified by the audience to come in, you will need
VBA. You can see an example of (not quite what you want, but it will give
you the idea) at my site at Example 6.7:
http://www.loyola.edu/education/PowerfulPowerPoint/
Click on "Examples by Chapter" and "Chapter 6."
--David
--
David M. Marcovitz
Microsoft PowerPoint MVP
Director of Graduate Programs in Educational Technology
Loyola College in Maryland
Author of _Powerful PowerPoint for Educators_
http://www.loyola.edu/education/PowerfulPowerPoint/