What is the woman studying?

M:What are you looking for?
W: We’re studying the basic structure of molecules.
M:Just any molecules? There are quite a few of them!
W: Yes, far too many. We’re only working with certain molecules that contain carbon and hydrogen.
M:What do you expect to learn from that?
W: We’re trying to see the kinds of carbon molecules in which we can use silicon as a substitute.
M:What difference would that make?
W: Silicon has a lot of the same properties as carbon, but it’s much more common than carbon.
M:Oh, I see.