The five ways and the five second dismissals
After Aquinas formulated his "Quinque Viae," students would often spend weeks studying each "way," as an introduction to these ideas.
Today, students spend five seconds having some professor guffaw, "Ha, if everything has a cause, what caused God?!" (Showing the professor himself has no idea what the second way claims.)
But to take this argument seriously, to spend the time necessary to grasp it, would start to produce doubts: maybe Aquinas was on to something! And we don't want that.