Avoid foo/bar. That is just confusing and cat/dog is as many characters and something people will have an easier time getting their heads around.
I taught at a bootcamp for six months (post graduation) and the biggest takeaway I have is that not everyone learns the same way. I never actually found a great solution for this, but it became painfully obvious that "easy" concepts for me were not easy for everyone. I believe visual learners have the hardest time with programming concepts. YMMV with a framework vs. "what is a variable", of course.
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