Just a quick post for a large topic here, because it's covered well in other places. Critics of evolution (almost entirely made up of religious, anti-rational crazies) have certain mantras they like to repeat often; one of the most common of these is "evolution is just a theory!" ... Apparently this is supposed to discredit it, and portray it as somehow being in serious doubt.
If only they knew what an idea has to go through in order to become a scientific theory. They are actually confusing "theory" with "hypothesis", the latter being a far weaker position prior to testing. There is very little you can say to people who put forward this view - because it is itself ample demonstration that they don't think for themselves in a critical, rational way and are thus unlikely to be swayed by any argument.
You could try showing them some pretty convincing (and very cool) evidence.
But at the end of the day, elegance lies in simplicity - and I normally end up replying "so is gravity".