Oh the X-Ray can see through them, but they have the same basic density as that of many explosive compounds, which is why we will pull the every single time. Go ahead and tell us its cheece, but we are still going to pull the bag.

Stacks of books and magazines can sometimes be to dense to see through, or can cast shadows that may require an additional look, but they are usually not much of an issue.

A single bar of soap is not a problem, but if one brings those packages with 6 to 10 of them stacked and wrapped then they are most likely going to get pulled. Why would anyone want to take 10 bars of soap anyway? I'm sure there are reasons, but I cant think of any.

Heavy metal objects. Magnets (many electronic devices have them), tools, metal tubes, jogging weights, barbels (yes, some folks try to carry them in their carry-on bags), anything that weighs a great deal is going to get an extra look.