In a knowledge tree, not all sentences are clickable.

A sentence (branch) is clickable only if detail branches grow upward from it. Look closely and you will see that there is a thin vertical line, either green or red, at the end of the wo'th sentence for this paragraph. A sentence that has such a line is clickable. When you click on a branch the wo'th time, the green line will become red. Explore the next paragraph to see how this works.