Not only is there no comprehension of the difference between weight and volume, there is no understanding that the rest of the world is metric (or even that the rest of the world exists, for that matter). My conversation with a scientifically challenged TSO who wanted to confiscate my toothpaste:

him: This toothpaste is above the limit.
me: No. Look here - it is 90 g.
him: (nervous about my pointing to the toothpaste) Don't touch it! ... Is 90 g more than 3 oz? (More to himself than to me.)
me: Toothpaste sinks in water, and therefore is more dense. As a result, this toothpaste is under 90 mL. (OK, I should have just said yes, but I tend to like precise answers.)
him: (clearly very confused) I'd better call a supervisor.

After a long talk with the supervisor regarding the density of toothpaste, the fact that it is sold by weight where I live and a swab for trace chemicals (the only one I ever saw done by the TSA), me and my toothpaste were finally cleared to go.

By the way, I am terrible with US-used measurement units, but should it not be 3.4 fluid oz, to differentiate from just oz used for weight??