One of the things that sets science apart is the way that it operates, building on the work of others and accepting, rejecting or altering understandings as new data come to hand. The idea that science is so open to change seems to be one of the hardest things to get across, in the classroom & in society at large: there seems to be constant surprise that scientists might alter their conclusions on an issue in the light of new information, and that each generation of scientists builds on the work of those who have gone before them.
That last point is perhaps exemplified by this quote from Isaac Newton: 'If I have seen further it is by standing on the shoulders of giants' (Newton was writing to his colleague, Robert Hooke, at the time, & I have heard it said that he was being rather snarky; Hooke was a small man.) Now Orac has a wonderful post about stepping back into the past in the medical world - how much of what doctors do today is dependent on the discoveries & advances of previous generations. You could substitute 'scientists' for 'medical doctors' & the message would be the same. [Warning: there is a graphic description of a pre-anaesthesia operation on a malignant breast tumour - perhaps not best for reading over lunch...]
(Orac's post could be a good basis for a classroom thought experiment; some of his commenters give some good supporting reading material, in the form of science-fiction novels based on the back-in-time premise. Now, if I could only think of a way to fit this into my lectures...)