Expressions can include pieces that are revealed via a mouse click. This is accomplished using the \toggle...\endtoggle macro. For example (x+1)^2 = \toggle {\rlap{?}\phantom{x^2 + 2x + 1}} {x^2 + 2x + 1} \endtoggle produces \[ (x+1)^2 = \toggle {\rlap{?}\phantom{x^2 + 2x + 1}} {x^2 + 2x + 1} \endtoggle \]