When Joseph Mastropaolo first made his claim that evolution was too improbable to be real science, it reminded me of Sherlock Holmes' axiom, that when you eliminate the impossible, whatever is left - no matter how improbable - must be the truth. I thought it a fitting rhetorical response to Joseph's rhetorical claim, so I used it a couple of times.
"Why is 'creation scientifically impossible?'"
Again, that claim is mostly for rhetoric. However, since creationism cannot be verified or refuted scientifically, then for all intents and purposes it is scientifically impossible.
"If it is, how does 'evolution must be true' follow?"
Creationists claim that there are only two alternatives: evolution or creation. Thus, by implication, if one is impossible, the other must be true. I'm simply using their own words against them.
Kevin L. O'Brien