2 other subtopics:
First science / statistics, how the rates of identical twinning are constant across all races, maternal ages, etc. It's a fairly constant and random occurrence. How scientists don't really know how or why it happens, how if the zygote divides days 1-3 the twins are diamniotic - dichoriconic, days 3-8 (I think) twins share a placenta, days 9-13, twins share everything. One good explanation of this is apartment living. di-di is like each baby having an efficiency apartment. They make great neighbors. mono-di is like a 2 bedroom apartment, each babe has their own bedroom, but they share a kitchen, which can sometimes lead to problems. Finally, mono-mono is like being cramped in a studio apartment. Tight quarters.
Then discuss how although identical twins share the same DNA, environmental differences can turn on or off certain genes, which make the twins less identical. Like my girls. They were born a pound different in weight. Who knows why. But the different in utero enviroment lead to differences all through their 2 years - the smaller girl has never caught up and is still a light eater. And radically different personality, but that's another matter. I think, anyway.