Orbital mechanics have a funny way of making an object return to its point of egress.
I think this is only true in a 2-body problem
One thing I have seen students get hung up on is:
x = x + 1
It looks like an equality statement that couldn't possibly be correct. Just telling them it's "an assignment statement" doesn't seem to penetrate.
have you considered switching to Haskell?
"Card readers? We don't need no stinking card readers." -- Peter da Silva (at the National Academy of Sciencies, 1965, in a particularly vivid fantasy)