In RIVAL, they're dissecting things! Originally, I had Kathryn and John dissect cats, because that's what I dissected in my high school anatomy class. That's right, we dissected cats. I'm not sure where they came from--I like to believe they were strays that were painlessly euthanized. I'm not even sure I was all that horrified when we did it; I just wanted an A, and I was able to separate the soggy stiff thing on my slab from my kitty cat at home.
(I won't even go into the field trip we took to see real cadavers - a few select students were able to sign up for a team that got to help dissect those. Not sure I could have done that, even for an A+++++.)
When RIVAL was in revisions, my agent requested I change the dissected animals from cats to fetal pigs. She thought it might offend people to read about cutting open animals that are commonly kept as pets. So I changed it, and it's cool. I don't think the book rises or falls on what my characters are dissecting. But I do think it's an interesting topic!
How about you? Did you ever have to dissect something in school? What was it? Were you grossed out? How did it go?