The boxes come in different colours to indicate different content: green for theorems, brown for formulas, and blue for examples. (Examples are also marked with a pencil icon to indicate that the student will need a pencil and paper to work them.) Behind the theorem boxes is usually a proof, behind the formula boxes, a derivation or discussion, and behind the example boxes the worked details. Or there may just be a comment, or . . . - the idea is that a raised box indicates there is something more behind its contents that the student should look at.